Transmission Fluid and Gear Oil

Automatic, CVT or manual: three fluids that do not swap

Transmission fluids are chosen by the manufacturer's specification, not a generic name. Conventional automatics use ATF to a stated spec, CVTs need dedicated CVT fluid, and dual-clutch gearboxes have their own. Putting the wrong one in can cause slipping, shuddering or wear. Manual gearboxes may take gear oil, ATF or even engine oil, depending on the design. The owner's manual, or the fill-plug and dipstick marking, names the exact requirement.

GL-4, GL-5 and the limited-slip question

API GL ratings describe the extreme-pressure additive package in gear oil. GL-5 suits hypoid differentials, but some manual gearboxes with brass synchronizer rings are specified for GL-4 because aggressive additives can attack soft metals. Clutch-type limited-slip differentials need friction modifier, either blended into the oil or added separately. Viscosity, such as 75W-90, is in the spec too. Some listings in this aisle are service helpers like dyes and cleaners, so check the title.

Questions about Transmission Fluid and Gear Oil

Can I mix different automatic transmission fluids?

It is best not to. Fluids differ in friction characteristics and additives, and a blend may not meet any specification. If you must top up, use the spec in your manual and plan a full fluid change soon.

Is gear oil the same as transmission fluid?

No. ATF is thin and contains friction modifiers for clutches and bands. Gear oil is heavier and built to protect gear teeth under high pressure. Using one in place of the other can damage the unit.

How do I find which fluid my gearbox needs?

Check the owner's manual first, then the fill-plug or dipstick marking. Sealed units may list the fluid in a service manual. Match the written specification rather than the color of the fluid.