Manual Transmission Fluid, GL-3 vs. GL-4
 

So I'm looking into what transmission fluid I want to use and it seems like everyone is recommending a GL-4 spec fluid, when the shop manual for our cars calls for GL-3.

I've been reading the sources on this post:

http://www.ft86club.com/forums/showp...77&postcount=9

to try and understand why GL-4 might be better and is ok to run in our cars but I'm not really seeing anything and hoping some one can explain it to me.

Here is the spec from the shop manual for reference

Attachment 124301

So that post essentially explains GL-4 vs GL-5. The difference between GL-3 and GL-4 is that GL-4 has a higher lubrication requirement spec than GL-3. And by that I mean there is equipment out there specially designed to test gear oils and simply put a GL-4 oil will sustain more load carrying capacity than GL-3. GL-4 will have more EP additives than GL-3. You can actually see that a substitute product on the list says GL-3 or later. GL-5 just can't be used because the EP level is so high it degrades the copper alloy parts in the trans (aka syncros).

GL-3 is an "inactive" spec according to the API which is why they are difficult, if not impossible, to find.

https://www.lubrizol.com/DrivelineAd...arOil/GL5.html

GL-5 can be used if it meets both GL-5 and GL-4. Motul Gear 300 meets both and likely achieves this with the use of metal deactivators.

Gear Tribology and Lubrication Part I
If you're not comfortable with this, run a gear oil that meets only GL-4 as it has 30-40% less extreme pressure additives. I do believe that a lot of people run Motul Gear 300 in the tranny although YMMV, of course.

@bluesubie I respect your opinions on oils/lubes, and would like your input on the following:


Royal Purple recommends their MaxGear 75W90 lube for transmissions/transaxles that spec a GL-4 or GL-5, but not for a GL-3. For a GL-3 application, they actually recommend their HPS or XPR 10W40 motor oils (not to be confused with the API certified oils). Presumably, this is because of the higher levels of zinc/phosphorus, which should help take the place of some of the EP additives in a GL-5 or 4. I'm thinking about giving this a try on my next drain/fill. What do you think?

When I used RP's Lubricant Advisor, it actually recommended Max Gear for the transmission. My guess is that the recommendation of HPS or XPR for GL-3's is for the old cars that originally specced GL-3. Where did you find that?

I know that many GL-5 oils use very little zinc and the main EP additives are phosphorus and sulfur. The question is how much sulfur is in HPS or XPR and how much is good enough for the twins, or does the high zinc and phosphorus do the trick?

Molakule at bobistheoilguy would probably know the answer. RP themselves would probably be helpful as well. As far as the viscosity goes, a 40 grade engine oil is 12.5-16.29 cSt's @ 100C and a 90 grade gear oil is 13.5-23.99 cSt's @ 100C so you'd be on the light end in regards to viscosity.

Here is a link to the cross reference chart:
http://www.royalpurpleconsumer.com/a..._Reference.pdf
When I first started looking at fluids, I thought MaxGear was the right stuff until I got digging a little deeper. Looking down through the list, they recommend their non-API oils for quite a few applications, not just the GL-3 spec 75W90.


I've tried quite a few different fluids, and mixes in the pursuit of happiness. Other cars may be different, but I have found mine to be sensitive to viscosity. Redline MT-90 was pretty much the same as stock, and their MTL (old label 70W80) was better, but not good enough. So far, I have found Pentosin MTF2 (75W80) gives the best results in shift quality. A ~60/40 mix of Redline MTL/MT-90 is a not too distant 2nd. I'm happy with the Pentosin, but I can never leave well enough alone. If there is a less expensive option that works as well, I'm all for it. I'm curious, and may give it a try anyway just to see.


Pentosin states a KV of 7.7mm2/S @ 100 degrees C, but I'm not sure how to convert that. VI is 177.

I would contact RP and see what they say. Most likely they would recommend Max Gear since that's in the product advisor. I think the HPS/XPR recommendations are specific to those applications listed.

mm2/s is the same as CsT's.
http://www.endmemo.com/sconvert/mm2_scst.php

I was going from the top of the chart where it stated 75W90 (and 80W90) GL-3 applications cross to the HPS/XPR, and that GL-4 or 5s would be the MaxGear. The owner's manual for my FR-S specs a GL-3 fluid. I saw that and didn't even look at the product advisor. I'm temped to call, and see what kind of reaction I get when I point out the GL-3...they'd probably think I'm an a-hole, but I genuinely curious.


I didn't know that mm2/s Cst's were the same...should join up over at BITOG, but I spend too much time here.


Thanks for the link.