Gear Oil (Transmission Oil)

It is thicker than engine oil and graded with codes such as 75W-90. API GL-4 and GL-5 describe performance levels; the wrong one can harm synchros. Some gearboxes are filled for life, others have change intervals. Many differentials need hypoid oil, and limited-slip units may need an additive. Dual-clutch and automatic gearboxes use their own specific fluids, not ordinary gear oil. Filling to the bottom of the filler hole is a common way to set the level on manual boxes.

It forms a strong film between gear teeth under very high pressure.

Low or old oil causes noise, hard shifting and wear. Check for leaks at seals and drain plugs.