TT, I'm not an oil expert. I try to follow the advice of experts with experience with our cars. From what I took in some of the threads, there is one in particular that Need4speed mentioned, Brian seem to hint that the 75w-90NS was what he uses. I could be wrong....maybe he will chime in. I wouldn't mind using a lighter oil like the MTL but how is on gears? What do you focus on gears or synchros? That is the pressing question. Is MTL bad on gears over time? If so, what's more expensive to replace....gears or synchros? Does GL5 wear out the brass in the synchros ...does GL4 teardown the gears???