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Atf in a manual transmission


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I just finished a 5 spd conversion on my '88. I noticed that the shifting was stiff and clunky especially when cold. I used 5.1 qts of 75-90 gl4 as recommended. I was told years ago from a mechanic friend that he uses atf/Dexron in most manual transmissions and gets excellent results. I tried it on an older BMW a long time ago and remembered that the results were noticeably improved.  So I dumped the gear oil out on my pathy today and filled it with Dexron. The results are NIGHT and DAY! Shifts like butter now. I'm actually amazed by the results.

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20 hours ago, octanevortex1 said:

I just finished a 5 spd conversion on my '88. I noticed that the shifting was stiff and clunky especially when cold. I used 5.1 qts of 75-90 gl4 as recommended. I was told years ago from a mechanic friend that he uses atf/Dexron in most manual transmissions and gets excellent results. I tried it on an older BMW a long time ago and remembered that the results were noticeably improved.  So I dumped the gear oil out on my pathy today and filled it with Dexron. The results are NIGHT and DAY! Shifts like butter now. I'm actually amazed by the results.

Are you sure the old fluid wasn't shot or the wrong kind? If it were me I wouldn't risk it. People get many miles out of those transmissions with properly rated GL-4 and good maintenance schedule.  Not all transmissions are the same.  It will definitely feel like it shifts smoother with the thinner stuff in it but that doesn't mean its lubricated right.  I would look into the type of ATF your using and at least make sure it wont react badly with the yellow metals in the transmission like the synchros.

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I used sta lube GL-4. I finished the conversion about a week ago so the fluid that I drained out was fresh. I replaced it with Valvoline Dexron 3. I did a little research on the effects of atf on the synchros and didn't come up with much regarding damage/corrosion. I'm sure that a properly maintained trans with GL-4 will last a long time. I prefer to follow the factory recommended lubricants,  but just couldn't handle the stiff shifting for the first few miles and the atf completely eliminated that. 

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I've read that synthetic gear oil shifts better in the cold than the standard stuff, but I don't think I've heard of someone switching one of these to ATF. Some manual transmissions do run ATF, including the Tremec in my dad's hot rod. The transfers in these can run ATF (that's what's in mine right now), so if it's good enough for those gears and bearings, seems like the gears and bearings in the trans should take it just as well. I guess the question is how it treats the syncros. Might be a question to research on BITOG, if you're looking for a rabbit hole to fall into.

 

If you decide to stick with it, check back later and let us know how it's holding up!

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