I just bought a new-to-me '99 Miata, very nice condition no pictures yet, since a true Bitoger worries about fluids first. The vehicle has been well maintained, but the manual gear fluid and differential fluid need to be changed. For transmission oil, the OM states to use a GL-4/GL-5 75w-90, and for the differential, the manual only states GL-5 (no grade given). Any suggestions? I've had good success running Redline fluids before in a number of vehicles. The car will only be driven in warm weather and garaged in the winter.