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Another thread mentions using a 50 weight oil in the transmission and transfer case. I simply used 80-90 weight gear oil in mine same as in the differentials. I don't remember where I got the info to use the gear oil in the trans and transfer case, but it was either in the manual or from people here. Am I wrong to use gear oil in the transmission and transfer case or is this another one of those 6 of one half a dozen of the other debates? Just need to know, thanks.

Every vehicle I've ever owned over the last 50 years has had SAE 90 gear lube in their manual transmissions. I don't think running gear lube in your transmission and/or t-case would be a problem. Essentially, a transfer case is nothing more than a 2-speed transmission with one input shaft and two output shafts.

I use straight 50 synthetic oil. shifts a lot better, especially cold

bpj911 wrote:I use straight 50 synthetic oil. shifts a lot better, especially cold

Yes, it would! 90-weight is like glue at anything below freezing. It takes a couple of miles or so to thin it out enough for easy shifting.

Nickathome wrote:Another thread mentions using a 50 weight oil in the transmission and transfer case. I simply used 80-90 weight gear oil in mine same as in the differentials. I don't remember where I got the info to use the gear oil in the trans and transfer case, but it was either in the manual or from people here. Am I wrong to use gear oil in the transmission and transfer case or is this another one of those 6 of one half a dozen of the other debates? Just need to know, thanks.

Most transmission builders these days are recommending 30, 40 or 50 weight heavy duty engine oil in their boxes; even in their older units built many years prior. This goes by ambient temps with 30 weight being recommended for colder climates. Shifting gears is much easier in cool weather, the entire unit will run at a much cooler operating temperature. Some of the 80-90 gear lubes that are recommended for differential use have ingredients that are harmful to brass parts, most of the older M37's have brass parts in the transmission synchronizers. If the 80-90 gear oil you are using has this additive & your transmission has brass parts, problems with part failure will result down the road at some point. The 50 weight flows much easier to critical parts that may be oil starved upon start up until 90 weight warms enough to flow freely, it's just a much better choice all the way around. The only components that need 80-90 are the differentials, the winch end housings, & the steering gear box.

Thanks, Charles. Once again, you have come through with a good recommendation backed with a logical explanation. I, for one, am glad you're willing to share your experience with us!

I'll second that!!

I use synthetic 10w30 in my Pinzgauer transmission. makes it shift so much better and is quieter as well. I have had a lot of people tell me I will wreck the trans doing that as it calls for 90wt. I drain it yearly and all looks well. I think I will chance it.

Using GL-4 instead of GL-5 and you ought to be fine.

CD

I kinda thought like Lifer 90 wt was what was always used. Got it in both the trans and the transfer case. So I should change to a straight 50wt in the trans how about the transfer case??

We have been using straight 50 weight in transmissions & transfer cases for a number of years now, great results. I would recommend Chevron RPM Delo single grade 50 weight.

Thanks Charles

Charles - thanks again. We found the "engine oil-heavy duty 30W" at NAPA for the transmission. We figure we would run it on the 30W for awhile just to help "wash" the old stuff out - then change later this summer after a few hours. We have the 90W GL for the pumpkins. We put in the 5qts of 10/30W engine oil and noticed an immediate change in rpm during cranking!! You could tell the engine cranked a lot easier.

I'm glad to see that straight 50 weight is available again. I recall as recently as a couple of years back there were ads in Hemmings with guys looking for straight 50. It was also scarce when I served in the Army in the '70's and '80s.
Gary

I have just purchased a 51 m37 and have always used Amsoil and wonder is anyone knows which one to put in my trans and transfer case and how much?