ok for the rx8 mazda recommends a lighter oil than previously (5w-20 i think), its probably for fuel economy, but did they have to make any other changes to make it work? ive had no trouble running any of my cars on 5-20, but will i?

I run 50weight



yup.

I run 5W20 in a streetported '84 13B. People say it can't be done but I've had no problems whatsoever.

As long as it has oil pressure at idle, it will be fine. Anything thicker just wastes power to drive the pump, and dumps more oil out of the regulator instead of flowing it to the bearings. And rotors.