I use John Deere fluid. Was highly recommended on the turbo forums and yellow bullet. Tightens the stall a bit, but I've had no problems on the street. 440 w/ S475 Turbo & 727.

1969 Plymouth Road Runner (440 w/ Boost! RIP) now a low-deck 470 with hotchkis suspension, nascar boom tube exhaust, & big brakes.