You definably want to leave the spring in there, I would recommend picking up this spring set HPI makes just for the differentials. It comes with springs of different stiffnesses so you can try to see what one you like best. You could use Silicone oil but because the RS4 3 does not use sealed gear differentials the oil will leak out over time and make a real mess, not fun to clean up.

http://www3.towerhobbies.com/cgi-bin...?&I=LXZT47&P=7