different t5 use different fluid, world class use atf, non-wc use gearoil, i think. I do not notice any problems running synthetic 80/90 in it so it prolly don't matter
Later t-5 REALLY like Royal Purple Synchromax. It's made a HUGE difference in shift quality; can't vouch for the earlier ones. Synth flows better cold; the trans stops doin' that notchy thing goin' into 2nd.
If the trans you have is designed to use ATF, 80-90 will make for harder shifting.It will also take more horse power to turn with gear oil.
Depends on the T5. Some were built for gear oil and some were built for ATF. I can say that putting ATF into one designed for gear oil kills it pretty quickly. Seems like the synchros go out.
I am running 50-50 Royal Purple Syncromash and GO-90 mixture in my non world class T-5. Works well for me. Don Coatney