i see there are different weight oils for shocks and diffs. if you replace the shock oil weight would that make the shocks stiffer? and whats a good diff lube to use for Yokomo Drift Cars and what do heavier weight shock oils do? and whats a good cleaner for a RC car?

Offroad cars use 30-40WT oil, onroad use 60-80WT oil, but for drifting the spring is more important (unless you're using rubber tyres) so i'd say 40WT is fine

Kev