Short answer
Automatic, CVT, dual-clutch, and manual transmissions, differentials, and brake, steering, and cooling systems each need a fluid built for them. Use the specification in your vehicle’s service information for each compartment; engine oil and gear oil are not interchangeable.
Automatic, CVT, and dual-clutch transmissions
Automatic transmission fluid (ATF) controls shifting through its friction properties as well as lubricating. Manufacturers specify fluids such as DEXRON, MERCON, or their own approvals, and a transmission can shift poorly or wear if the friction characteristics are wrong.
Continuously variable transmissions (CVTs) and dual-clutch transmissions (DCTs) use their own fluid types. A “multi-vehicle” ATF is suitable only for the specifications it lists.
Manual transmissions and transfer cases
Manual gearboxes may call for manual transmission fluid, gear oil, ATF, or even engine oil, depending on the design. Synchronizers need specific friction behavior to shift smoothly.
Gear oils rated API GL-5 carry more extreme-pressure additive than GL-4. Some older manual transmissions with yellow-metal (brass or bronze) synchronizers specify GL-4, so check before using a GL-5 product.
Differentials and gear oil
Differentials usually use a thick gear oil such as 75W-90 or 75W-140. Limited-slip differentials may need a friction modifier, either already blended into the oil or added separately. Towing and heavy loads can shorten the differential service interval.
Grease
Grease is oil held in a thickener. Consistency is rated by NLGI grade, with NLGI 2 most common for chassis and wheel bearings. Thickener types are not always compatible, so clean out the old grease when switching types.
Brake, steering, and cooling systems
Glycol brake fluids (DOT 3, DOT 4, DOT 5.1) absorb water over time and need periodic replacement. DOT 5 is silicone-based and must not be mixed with glycol fluids.
Power-steering systems may use ATF or a dedicated fluid. Coolants use different inhibitor chemistries; follow the vehicle’s coolant specification and avoid mixing types unless the product says it is compatible.
Plan the whole service
For each compartment, record the required specification, capacity, fill procedure, and any gaskets, filters, or crush washers you’ll need. Keep the product codes with your maintenance records.
Frequently asked questions
Can I use gear oil in a manual transmission?
Only if the manufacturer specifies it. Manual transmissions may call for gear oil, MTF, ATF, or engine oil, and some need GL-4 rather than GL-5.
Is universal ATF safe for my transmission?
Only if it lists your transmission’s required specification. CVTs and dual-clutch units usually need their own fluids.
Do I need a friction modifier for a limited-slip differential?
Many limited-slip differentials need one. Some gear oils include it; otherwise it is added separately. Check the axle requirement.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].
