After doing my homework and checking with several performance transmission builders....the recommendation was for 70w gear oil in a NWC T5 and ATF in the WC boxes. Based on that, I've been running 70w in my 85 NWC .72 OD T5. The transmission has shifted with no issues. At 3K I drained the box and filtered the oil in a search for any brass from the synchros.....the oil showed no evidence of any metallic residue. I'm currently at 8000 miles with no issues.
Stuart mentioned "stiffness" in shifting and on several mornings when the temps got down into the 30's and 40"s you can feel the extra resistance until the engine/tranny heats up. However, the T5 has never ground between shifts, even on the coldest mornings.
I recently picked up a NWC T5 (brass synchros) from behind a 305 cid Camaro which had been run on ATF. In draining the box and filtering for metallic residue I found none. In opening up the box to make the hybrid conversion I noted that the synchronizes showed no wear/damage.
So what oil am I going to run in this NWC brass synchro hybrid transmission? 70w synthetic gear oil. However, this is primarily because our normal temperatures are higher than average, and I'm going to be running it behind a more powerful engine. Do I think ATF will lead to premature failure in a brass synchro NWC transmission? I don't think so, although the performance transmission builders disagreed.
I'm sure there are some out there that are running ATF in their NWC T5's that will have insights to add. One thing absolutely for sure....if you're running the synthetic synchro/needle bearing WC T5, ATF is the way to go.
Dave
