GM can't void the warranty if the fluids meet their spec. But how would they know ... unless RP actually is purple. (Is it?) Mobil1 meets the H3's GM4718M engine oil spec. Don't know if RP does.
http://www.elcovaforums.com/forums/s...ht=mobil1+spec
For the transmission, it probably needs to meet Dexron III-E or III-H (there's a difference and again Mobil1 ATF meets both). The diffs already contain synthetic oil so aftermarket synthetics probably wouldn't hurt but wouldn't necessarily help