like Bogartyr said, the synthetic is too slippery, and the synchros will burn up. As for dyno juice ruining a clutch? That's is so damn funny I almost wet myself. If the internals of a tranny have anything to do with a mating surface outside the transmission, then you have problems. I always run 10w40 in the summer, and 5w30 in the winter. Anything more than 30 and the tranny shifts hard in the cold. I swap trannies around like there's no tomorrow, and I've NEVER had a problem using Valvoline. I swear by it. Penzoil and other high parafin oils can gunk things up. So stay away from them. Other than that, any dyno juice should be good to go!