Which Gear Oil or Differential Oil should I use? Personal Conclusions and discussion. Molybdenum additive use?

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Which Gear Oil or Differential Oil should I use? Personal Conclusions and discussion. Molybdenum additive use?

Post by Toyotise »

I’m posting this to share in a searchable form my findings on preserving the drive train of the Tercel 4wd vehicles.
I’ve spent a lot of time on here, other websites and resources, and referencing the Factory Service Manual for the most correct answer to the question: Which differential oil or gear oil should I use? These are the facts and conclusions I came to. (TLDR: I am running Sta-Lube 85w90 GL-4 Multi Purpose - Hypoid Gear Oil for the reasons listed below. Will update this thread on how it works for me. Haven’t made a final differential oil selection I am happy to share with others yet).

These are the absolute undeniable facts:
Factory specifications:
Manual Transmission takes 4.1 qts of 80w90 GL-4
Rear Differential takes 1.1 qts SAE 90 wt GL-5

-Manual Transmission has brass components.
-Manual Transmission is integrated with (lower load design) hypoid type front differential (Manual transmission is really a transaxle).
-Rear Differential is a hypoid type with all steel components

Why you shouldn’t put GL-5 gear lube in a transmission with brass components:

https://www.widman.biz/uploads/Transaxle_oil.pdf
Simplified for TLDR: Sulfur-based protective additives In GL-5 rated oils bind to steel gear teeth and brass synchros to form sacrificial hard wearing layer. Not a problem for steel gear sets, as the bond of the steel molecules to itself is stronger than the bond of the Sulphur compounds to the steel. Under friction sulphur compounds are rubbed off. Minimal to no loss of steel occurs.

However the bond of the sacrificial sulphur compounds to BRASS components is stronger than the bond of the brass to itself. Under friction this causes a layer of brass to strip off along with the sacrificial coating. This leads to unnecessary wear of brass components.

The designers of the time did not engineer the transmission or differential components for use with synthetic fluids.

Synthetic fluid advantages: better resistance to thermal breakdown (maintain their viscosity range over a longer heat affected period), and better low temperature viscosity range performance. Require additives for extreme pressures in transmissions.

Petroleum (fossilized dinosaur juice) based products have a better cushioning effect (thicker), but less resistance to thermal break down over time. Require additives for extreme pressures in transmissions.

The correct additive package / rating is as critically important as selecting a product with the correct viscosity. (example: Don’t do something like blend use SAE 40 and SAE 50 motor oil to make your own 80w90 gear oil blend as it won’t have appropriate additive package).

The transmission/transaxle environment is relatively low temperature compared to engine oil operating temperatures. (Thermal breakdown over time of viscosity is less important).

The Tercel 4wd design use philosophy (use on unimproved [dusty, rough] low traction roads, and all weather conditions) meets the definition of Severe Service Schedule.


Conclusions:
1) Any of some oil is better than no oil. Check oil level frequently, keep level topped off, replace leaking seals.

2) The engineers didn’t decide on these oil weights and specifications arbitrarily, although we may never get access to the list of specifics.

3) Blending, selecting, or deviating from these guidelines is at your own risk and your mileage may vary. Ask yourself the question: do you really know enough to play petro chemical engineer? Be thoughtful and conservative in any experimenting.

4) Although synthetic oil‘s may look good on paper and be completely appropriate in more modern designs, there’s enough anecdotal evidence to suggest that for various reasons they may not be the most appropriate for the age and design of these transmissions. (Cushioning effect of “thick” Dino based oil).

5) Change fluid at frequent, regular intervals (10 to 20,000 miles). You cannot hurt anything by doing it too frequently. The purpose and design of the Tercel 4wd is for use in all weather low traction conditions and on unimproved roads. If you’re using the vehicle for its design purpose philosophy it’s going to meet the definition of the Severe Service Schedule automatically. It won’t hurt anything to change it too frequently (especially when you consider the cost, expense, and difficulty of rebuilding and/or sourcing replacement transmission).

6) Follow the correct procedure in changing the transmission fluid. Per the factory service manual (see FSM Page MT-70) drain from all three drain plugs, replace rear plug and back out eight turns. Fill 2 quarts from front filler plug, fill the rest from rear filler plug.

7) The majority of the transmissions out there have over 100,000 miles on them. That they are functioning as well as they are at their age speaks of the tremendous engineering and work ethic that went into designing, producing, and assembling them.

My personal selection:
I chose dino-based Sta-Lube 85w-90 GL-4 Multi Purpose - Hypoid gear lube for my transmission.
954C5F20-872B-4193-8024-A4B718A14BEE.jpeg
3A8AC737-4E81-4030-A85C-4D7A1FCBB0C1.jpeg
DE884B85-A51B-4BBD-A051-670D4CDAE412.jpeg
I like that it’s a Dino based GL-4 that is also suitable for Hypoid gear sets. I intend to trial run this for 1000 miles to evaluate its performance, and to inspect it’s condition/ quality and check for presence of wear metals. I live in a warmer climate, (normal winter lows in the 15’s-20’s Fahrenheit). I do not expect the difference from factory spec of 80w90 to 85w90 to be cause for concern considering the transmissions wear state of 250+ thousand miles.

For further research and discussion:
Molybdenum additive for gear sets manufactured by Liqui-Moly

https://products.liqui-moly.us/additive ... gears.html
I’m interested in the possibility of using this product and I’m interested in other people’s experiences or opinions. I haven’t found much research or anecdotal accounts done on utilizing this product in gearboxes. Molybdenum is an effective well known solid lubricant. Liquid Moly is arguably the longest running, most well known, manufacturer of such molybdenum additives. My hypothesis is that as long as the individual particles of molybdenum are fine enough to stay in suspension and not settle out/form sludge and-or clog up the transmission lube pump then it could be a useful additive to reducing friction and extending the life of these transmissions. Any opinions? Experience? Or other thoughts on this?
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Re: Which Gear Oil or Differential Oil should I use? Personal Conclusions and discussion. Molybdenum additive use?

Post by Petros »

I have used the moly additive in my transmission that has about 400k miles on it. the syncros are worn and I have experimented with various gear oils as well. I do have some professional experience with lubricants, so I experimented a bit to see if I can keep my old worn trans working for as long as possible. the only alternative is to keep beating it until I either bundle it up, or rebuild/replace with hard to find new parts. Here was my experience:

1) I tried all synthetic (Royal Purple MTF), mostly to see if I can stop any further wear on it, but found with my old trans it made the syncros act completely worn out, I got gear clashing with every shift unless I doubled clutched. it was also seeping more noticeably out of the seals. on a recent rebuilt trans with new seals and synros, this may not be the case, but it was with mine. I have replaced the rear output shaft seal and the shift level seal on the side of the gear box earlier, as well as the front axle seals a few years back, but I was getting a noticeable drip off the bottom of the with pure synthetic gear lube. could have been from the replacement seals, or front the front input seal what I have never replaced, and did not appear to have leaked before.

2) I changed that back to a 50/50 mix of synthetic MTF and regular Gl-4 85-90w and it stopped the gear clashing at shift and the leaks slowed down to a rare and occasional drip, which is what it was before I tried all synthetic.

3) on another change, I added some of the moly additive to the same 50/50 mix. It was supposed to improve wear without affect to the syncros, and it seemed to help it run smoother, but that was hard to tell. I do not know where to buy in the USA, I got two small tubes from a Canadian auto parts store. I would have stuck with this combo except I have not been up to Canada since and ran out.

4) another problem developed with this mix on very cold winter startups. when it is subfreezing, and I start off, the synthic is doing its job too well. when I first pull away, it is nearly impossible to shift into first or second, even with double clutching. it makes no grinding or anything, it is will not go into gear, I suspect because the oil is so thick, and with the slippery synthetic, there is not enough friction for the syncro to work. After I drive it a bit and the trans warms up this problem goes away, but usually I am in slower city streets right at start up, and was worried I may have a traffic problem on icy roads with it like that. so I considered this problem, and I know when I worked for a professional racing team for Nissan, we used to run a mix of 50/50 gear oil and ATF to improve syncro performance on the race car. it helped speed shifting (consider the tran main shaft into RPM was a high as 10,000 rpms). so I thought I would give that a try, but I am a bit leery of putting 5wt ATF in my old worn transmission, so I put 2 qts of GL-4 85-90w gear lube, one quart of MTF synthetic to lubricate well to reduce wear, and one qt of ATF. and that combo seems to work well, it improved the performance of my worn syncros, does not leak much, and it runs smooth and quiet. If I had any more moly additive I would put that in with it as well. but right now it feels like that combo will run my old trans for a very long time. still running well, no clashing, no whine, and over 400k miles on the original gear box and front diff.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
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Re: Which Gear Oil or Differential Oil should I use? Personal Conclusions and discussion. Molybdenum additive use?

Post by The Professor »

OP, the PDF you linked to doesn't take any position petroleum-based oil being more "cushy" than synthetic gear oil. The main difference between the two oil types is that (while both petroleum-sourced base stocks) "dino" oil is less consistent in the uniformity of the molecular chains of each particle of oil. This difference in length leads to a difference in how quickly the molecular chain breaks down under load and heat, shorter chains fail sooner, longer chains have more breaks to give before they've broken down to their base elements. Synthetic oil, on the other hand, every chain is the exact same length, the oil is uniform, and each chain will break down at the same rate. "Cushiness" is a result of molecular chain length. Redline Shockproof, in both flavors, have longer chains than traditional gear oils, for example

"Cushiness" discussion aside, I also went through the information labyrinth surrounding GL-4 gear oil and hypoid gearsets a few years ago. My conclusion, after reaching out to several fluid suppliers, was to run the Redline MT-90 synthetic gear oil. After a few thousand miles on the fluid (I don't drive the car often) I can report that while it hasn't COMPLETELY eliminated my 2-3 upshift grind, it has minimized it. The transmission has never been resealed, and this fluid isn't leaking a drop.
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Re: Which Gear Oil or Differential Oil should I use? Personal Conclusions and discussion. Molybdenum additive use?

Post by dlb »

Lots of different opinions on this topic, big thread on it here: viewtopic.php?f=2&t=9667
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Re: Which Gear Oil or Differential Oil should I use? Personal Conclusions and discussion. Molybdenum additive use?

Post by Toyotise »

Cushiness may not be the most technically correct term to describe the difference. What I was trying to describe is how at the same temperature, within the same brand, in the same rated viscosity, behave differently, synthetic vs conventional.

I was intrigued at the Redline series of product until I found the following information from around 2018 on a Volkswagen forum I used to frequent:
https://www.thesamba.com/vw/forum/viewt ... ?p=8821106
Specifically post at Sat Jul 07, 2018 10:04 am by user “gears”
Here's a sampling from around the U.S. VW industry:

A number of VW trans shops (like Transaxle Engineering) won't warranty their rebuilds if any synthetic is used.

Some gear suppliers state their warranty is void if Redline is used.
Go Westy has finally switched away from Redline GL-4 to the GL-5 camp.
German Transaxle confirmed with a phone call to Redline that their MT90 formula had changed (explaining why shift quality had deteriorated).

One of the oldest and most respected VW trans builders, Dave Folts, still recommends Swepco 201 exclusively, cautioning against any lube with moly additive.
Volkswagen transaxles are similar in that they have the integrated differential/ transmission (transaxle) design. Redline possibly was a viable choice in past formulations but there’s been enough recent anecdotal coincidence of transmission problems with recent use. Personally I’m a big fan of synthetic, I run the appropriate viscosity and GL rated AMS Oil in all our modern vehicles transmissions. I’ve noted a consistent 5-7% improvement in fuel economy in every vehicle I’ve ever switched to.

It appears that the Tercel transaxle (though possibly only due to coincident factors of age and high mileage) is sensitive to gear lube choice. Another hypothesis I have is that examples that have sat for a long time the lubricating film on the higher input shaft bearings Drains away due to gravity and time allowing corrosion to take place within the bearings and races. Since it takes a good bit of driving for new gear oil to circulate up to them when the vehicle is run after a long cessation of activity, premature wear can occur.
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Re: Which Gear Oil or Differential Oil should I use? Personal Conclusions and discussion. Molybdenum additive use?

Post by The Professor »

Interesting data point from GoWesty. I'm deep into the vintage Subaru world and our transaxles are basically mirrors of the air-cooled VW transaxles. Advantage Subaru, as they were designed for GL-5 since at least the 1980s.

The shifting in my Tercel hasn't degraded since I started using the MT90. Also, since Toyota in their infinite wisdom neglected to include a transmission dip stick, I won't be inspecting the fluid until it's time to drain/refill.

Regarding the myth of synthetic fluids causing seals to weep/leak, here is a single data point on the topic: https://www.youtube.com/watch?v=Pa4RwsiZUmI
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Re: Which Gear Oil or Differential Oil should I use? Personal Conclusions and discussion. Molybdenum additive use?

Post by Mark »

For all fluids, I use the specification that was recommended in the manual. The brand doesn't matter to me. I buy whatever is cheapest on sale at the time. Experimenting with different specifications based on internet rumors is asking for trouble in my opinion. I like to think that the Toyota engineers recommended a certain oil/fluid because its properties were most suitable for the equipment's materials and the conditions they are exposed to. Of course, there are probably conspiracy theories saying that the engineers recommended certain fluids (even though there are much better ones) because they wanted the cars to wear out quicker so you would have to buy a new one or maybe their uncle owned the substandard fluids factory.
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Re: Which Gear Oil or Differential Oil should I use? Personal Conclusions and discussion. Molybdenum additive use?

Post by Petros »

I notice that those saying they have not had any seals weeping when they used MT90. I am not familiar with that product, I have never used it, and know nothing about it.

I had used Royal Purple MTF, and I got noticeably more leakage from my existing seals on my transmission. This observation is not a rumor, but an observation of fact.

I have no idea if RP-MTF is a different viscosity or has different properties than MT90. The MTF was noticeably leaking from my trans, I could see the purplish drips under the trans, but when I used 50/50 mix of conventional gear oil and MTF, it reduced this leakage.

You may have had another experience with a different transmission and different gear oil.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)
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Re: Which Gear Oil or Differential Oil should I use? Personal Conclusions and discussion. Molybdenum additive use?

Post by The Professor »

Petros wrote: Tue Jul 20, 2021 4:15 amI have no idea if RP-MTF is a different viscosity or has different properties than MT90.
Google Search is a powerful tool... :wink:
Petros wrote: Tue Jul 20, 2021 4:15 amThe MTF was noticeably leaking from my trans, I could see the purplish drips under the trans, but when I used 50/50 mix of conventional gear oil and MTF, it reduced this leakage.
Sounds like your seals were already leaking when the synthetic went in, which is a BIG asterisk on this data point.
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1984 Subaru Turbo-Traction wagon & hardtop
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Re: Which Gear Oil or Differential Oil should I use? Personal Conclusions and discussion. Molybdenum additive use?

Post by Petros »

The Professor wrote: Tue Jul 20, 2021 10:22 am Sounds like your seals were already leaking when the synthetic went in, which is a BIG asterisk on this data point.
yes, likely. I stated so, seals were not original, but not brand new either. with conventional 85w-90 gear oil it was very slow and not a big deal. an occasional drip of a thick dark amber oil off the bottom of the trans. checking the level once a year or so showed no noticeable loss inside the filler hole. But with the royal purple MTF, I got a purplish small slick under the car with a constant drip. New seals all the way around may have reduced it. though I suspect worn bearings would allow the shaft to wobble a bit, so new seals my not stop it entirely. my trans has about 400k miles on it, anything is possible. adding seal conditioner, or a stop leak, that swells the seals may have also reduced the loss, but I am reluctant to swell the seals on a car I plan to keep, might work as a temp stop gap measure, but I would not want to drive it indefinitely with chemicals in it that cause the seals to swell up.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)
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