I have a 2002 subaru wrx. I want to change out the differential fluid and transmission fluid because I don't know when they were last changed. From what I've seen it seems that amsoil severe is among ine of the best and i can get that locally. My questions is they offer 75w90 and 75w110. Which would be better for the trans and diff? Would one weight be better for one and another weight for the other? The car is currently about 300whp, it is a daily driver and doesn't see track use but does get spirited driving.