I used the Redline stuff in mine for about three years. I never had any issues with it, but for some reason, the plastic in the input shaft bearing got brittle and led to bearing failure. Makes me wonder.

I've heard that, on a NV-T350 that isn't pushing above 200hp through it, you can try the synthetic stuff, but I no longer see the benefits. I've got 180,000 miles on mine and the only wear my internals show is due to chunks of bearing flopping around inside the trans when that bearing failed.