A differential lets an RC car's driven wheels — or its front and rear axles — turn at different speeds through a corner. Gear diffs are tuned with silicone fluid; ball diffs are tuned by tightening.
Thicker fluid tightens the diff so both wheels act more alike; thinner fluid lets them break apart more easily. Broad starting ranges: a 1/8 buggy commonly runs 5,000–7,000 cSt front and center and 3,000–5,000 cSt rear; a 1/10 4WD off-road car roughly 5,000–10,000 front, 5,000–7,000 center and 2,000–5,000 rear. Tighter diffs suit high-traction tracks and put power down harder off the corner; looser diffs help the car rotate on low grip. Change one diff at a time and log the cSt — unlike shock-oil “weight,” cSt is a real viscosity unit. Full-size race cars run differentials for the same reason but tune them with clutch packs and lockers, not a syringe of silicone.
Diff fluid is one of the biggest single-change handling swings on an RC car, and getting it wrong either pushes the car on entry or spins the inside wheel off the corner.