My personal choice.....based on 14K miles on my S10 based 1986 NWC (brass synchro).72 OD T5.....all with 90w gear oil in my 54 truck. No evidence of metal in case at 3K, 6K and 10K. In my Camaro hybrid T5 behind a built 355 in my hot rod, also with a NWC (brass synchro) T5 I run (3K) Royal Purple SynchroMax with no metal in oil sample.
As Wrenchbender pointed out, the WC box with the synthetic synchro, requires an ATF weight lube. My personal experience has verified that a used S10 NWC T5 box in good condition, will function properly with either 90W OR ATF. However, I would suggest using a fortified ATF if you go that route.
The only downside I've noticed with the 90w is a little stiff shifting on mornings when the temperature is in the 40-50's before the engine has warmed up. As for the Camaro-based NWC T5.....it has been put through the wringer behind the 355 and has proven to be a whole lot more durable than I had imagined.
As for shifting better; I'd venture to say that aside from being a bit stiff in cooler weather the 90w would be great in a box behind a 6 cylinder. Behind a V8, using the nwc Camaro box I wouldn't hesitate to use a fortified ATF.
Hope this helps.
Dave
