on a 91 thunderbird supercoupe - 70k miles - now should I change my oil the first 250 miles or 500?? I have heard both... I put castrol gtx in it I was also thinking about using mobil 1 then. the only advantage of having a aynthetic oil over dino is the breakdown is alot better with synth??? Will dino even breakdown? I know a supercharger will make the engine hotter... One last thing... what is a good viscosity to use? Right now I am putting in 10w30 I probablly shoudll have wen't lighter...