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Yeah, I wonder how much that shop guy really knows or understands about gear oil. Gear oil viscosity and motor oil viscosity are not comparable. 75W-90 gear oil is about the same as 10W-40 motor oil.

Here's a basic primer on the difference: http://www.widman.biz/uploads/Transaxle_oil.pdf

Quote
Viscosity:
First it should be noted that the SAE motor oil viscosity chart (J300) is different from the SAE gear oil
chart (J306). A SAE 40 motor oil has approximately the same viscosity as one of the thinner SAE 90
gear oils, while the SAE 50 Motor oil is similar in viscosity to the thicker SAE 90 gear oils.


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Originally Posted by 270winchester
I just bought a rebuilt G52 5 speed manual trans for my 1985 Toyota 4X4 pickup and the shop guy said to use 10W-40 engine oil in it, NOT 80/90 gear oil. He said when it's cold outside and the gear oil is cold, if you push the shifter hard you can actually break the aluminum shift fork from the resistance of the thick oil.

He also said NOT to run synthetic because it's actually too slippery for the synchros to work right.

My Chilton manual recommends GL-4 or GL-5 75/90 or 80/90 gear oil.

I'm going to use the 10W-40 because the shop will warranty the trans with that oil. They wrote on the side of the trans case by the check bolt in sharpie "10W-40 motor oil."


Your guy is out to lunch on both counts. 80W90 gear oil is a hypoid oil and contains sulphur, which is detrimental to brass synchros. That's the real reason not to use it. His second theory about synthetics being too slippery is equally bizarre. We have used it for over 20 years on everything from the current models going all the way to Porsche 356's circa late 50's without any problems.

Last edited by badger; 10/25/15.

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Originally Posted by Kodiakisland
Originally Posted by seal_billy
Thanks badger, that's what I was looking for. I was also gonna do the rear diff in my jeep cause I changed it a couple of years ago and put 75w90 royal purple in it and read after I changed it that if you tow with it to use 75w140. I pull a boat with it so I may try their diff lube also, if I can find it. I may just end up filling it with shell spirax.

Thanks
T-roy


According to ECGS, never, ever use Royal Purple in your diff. All they do is gears and have a pretty good idea what works and what doesn't. They recommend Lucas oil, but Royal Purple is the only brand they say not to use. I use Lucas 85-140 in my Tacoma.


Yep, have heard the same. I'm gonna drain it and refill with 75w140 before it gets cold. The data 30 in the YJ is the week link and I sure don't want to do anything to help it fail. Thanks


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Weak not week, just in case the spelling correctors show up.


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Well crap. My Toyota factory service manual says to use API GL-4 or GL-5 75W-90 oil.

From Red Lines website concerning their MT-90 75W90 GL-4 gear oil:
"Appropriate coefficient of friction for most manual transmission synchronizers (other's synthetic gear oils are often too slippery for proper synchro engagement)"


Last edited by 270winchester; 10/28/15.

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