diff oil and shock oil is silicone oil. the viscosity (sometimes in WT's) if how much it resists to flow. Basically, the more the WT, the thicker the oil. So 40WT is thicker then 20WT for example. There are other ways or measuring the viscosity but its the same principle.

in a gear diff, the oil slows the action of the gears. Diffs in rc cars are 'open' diffs which means that power is put to the wheel with less traction. So using thicker oil counteracts this and the diff feels more "stiff" giving more power to the wheel with traction..

if youve ever taken a real fast and shard turn, one wheel might have "ballooned". Thats because the diff too easily gave power to the wheel with less traction, and the centrifugal force made the rubber expand.

Same thing with shocks, thicker oil slows the shocks and thinner oil makes the action quicker, more 'bouncy'.


I dont know what oil to use in a firestorm so ill let someone else add that in.