When in doubt, I most often go by the users manual, but my 95 Geo Prizm states oil change 3000 / 3 months 10w30. I know this is old days thinking, but the car is only 20 years old. Did the gm and Toyota engineers really not know or was the oil that bad back then? Fyi, at 65mph, the Rpm is over 3000 and it only hold about 3.2 quarts of oil (w filter change). I guess the only way to really know what is a correct OCI is through UOA. I have Penz ultra 5w30 in now. Your thoughts.