is there any reason I cant run 75-90 hipoide gear oil in my 4 speed stick trans? on my 82 300D

The haynes manual calls for type a automatic transmission fluid in their four speed manual. Only reason I could visuilise changing to heavy petroeum based oil like your thinking about would be to try to lessen noise in a failing transmission. Perhaps has too much viscosity for your syncro rings to deal with as well plus the additional chance of changing wear patterns is not good either. Also will be pretty stiff in cold weather plus increase drag to some extent. The transmission seals may be designed for automatic fluid and the oil based fluid may swell and deteriate the seals as well. Overall just do not think it is a good ideal in general. Transmission capacity is only 1.7 us quarts so should not be a financial thing.

Also, some fluids are made to work with brass better than others... so the synchros last longer with the proper fluids...
Search for redline with JimSmith as poster....he has explained this stuff...

If you do it, you won't be able to shift it on a cool morning, not to mention that you will cause acclerated wear due to excessive heat and pour lubricant flow.

My '84 240D has 523,000 miles with the original transmission that has never been apart. It's never had so much as a seal replaced. It has had 10W30 in it since about 50,000 miles and changed about every 100,000 miles.

If you decide to use ATF, make SURE that it is Type A. The seals in that old transmission will not handle Mercon/Dexron. It will wear them out and cause them to leak.

Good luck,

i assume that you are thinking of doing this because your tranny is worn? perhaps the synchros are not working well? i recently put 15w40 in one of mine that the synchro was weak in and i am happy with the results. i am not sure how 8090 compares in weight but i think i have heard it is about like 30w. i dont think anything dire will happen if you try it out and if it is too stiff you can simply drain it out and put in something lighter. i am not aware of different types of rubber being used in seals. if it is leaking out the seals this will help a little but changing the seals is too hard and you will eventually need to do that.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

When Chrysler developed their new manual transmissions way back... I think they were called " new process" with tigher tolerances... they actually had some fires caused by using the old ' hypoid' gear lube... when old mechanics were not reading the instructions on the new machines... or did not believe them....
Still suggest you find JimSmith's explanation about MTL by redline... other good posts in that thread also...

I wouldnt put anything but "Redline MTL" in my MB trannys

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

i meant to say changing seals is NOT too hard and would need to be done eventually.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

OK thanks guys I'll try the redline MTL. I know that forklifts use ATF and my 1950 ford,and 54 mercury have 75/90 gear oil in their trannys with overdrive.
just wanted to see If any one exparimented with the oil thing. I have drivin
my 50 ford for 26 yaers cross country and never had to overhaul the trans in it.she likes 70mph and gets 21mpg all day with a 95hp flathead 6