In my Tremec 3550 and then my close ratio 600 box in my SPF, i have used the recommended ATF, in Redline, D4ATF. It worked well. But i wanted to try just something different, and have used Redline MTL and MTL 90 in other cars, with improved synchromesh shifting, but have found that the best of them all was a BG products fluid, a
synthetic called Synchorshift II, not cheap when i have my local repair shoop, who used other BG products in his work, to order it for me. I think it is about 20 bucks a quart, and takes just under three quarts. I drain my transmission from below, then put the drain plug back in, then run a plastic hose, the kind you can get at any home depot type store, i think a half inch, and stuff one end into the fill hole, and run the end up along side the passenger side of the motor, tie it up, and stick a funnel in it, and drizzle the gearbox
oil in until it runs out the fill hole. The warmer the
oil lthe better it flows in.
The D4ATF was really good, the BG just a tad bit better in shifting. These are still manly gearboxes, and take a hefty shove to shift.
I found on a hot race track that at the end of a long track session, my gearbox could be beat on 2-3 upshifts, but it worked fine when it cooled down, so i granny shift when it gets like that. Dennis Olthoff told me the Tremec isn't really a racing transmission, and with out a cooler, it just does that when hot.
so, i suggest either D4ATF or the Synchroshift. I have about 130+ thousand miles on my two Tremec boxes with these, and no problems.
hal copple
SPF South Carolina