I have a 1972 corvette with original turbo 400 trans and only 20,000 original miles. I know it is original miles because it belonged to a friend of mine since childhood who rarely drove it and bought it new. I was there when he got it in '72. He recently passed away and I purchased it from his estate. I do not believe the trans fluid has ever been changed and for sure not in the last 25 years. Trans performs with no problems but not sure if I should leave well enough alone or change the fluid due to age and possibly cause a problem after changing it. I would not flush it just drain out old and put in new. The current fluid on the dipstick looks and smells as it would have when new so no problems there. It would have Dexron in it now and would replace with current dexron6. which is backwards compatible. What would be the best thing to do ... leave it alone or drain out old fluid and replace with new dexron6 fluid??? Same applies to the differential which is positraction. Do I change fluid or leave it be???
change it all
For the transmission, if it really only has 20,000 miles, you should be safe to change the fluid. Just keep it if it does weird stuff afterwards, and change it back to the original fluid. My 1979 Catalina seeps a little transmission fluid and has over 100k miles (not sure exactly how many, the speedometer assembly needs repaired). I plan on pulling it and replacing the input shaft seal at some point as well as the other external seals, but I check it periodically and add a little fluid as needed. I use Valvoline Dex/Merc, which is compatible with the Dexron III it came with. I've not experienced any slipping. Honestly, after 43 years it shifts better than my 1999 Ranger does after I had it's transmission rebuilt.
As for positrac, I'm not sure on that. I would say you would be good to change it if it wasn't abused, since a LSD has clutches and needs friction modifier in the gear oil to operate. If the clutches are in good shape, you should be good to change it. I'd recommend getting a digital copy of the GM Dealer's service manual for your 'Vette, they're on RockAuto for $25. I bought the manual for my '79 Catalina and it's like my Bible when I need to replace stuff. I pull it up on my Microsoft Surface as I work on the car, probably the most handy $25 I've spent recently.
