How to find the right Gear Oils AI guide

Written by an AI model from its trained knowledge.

What the AI looks for in Gear Oils

API GL rating: Hypoid differentials typically need GL-5. Many manual transmissions specify GL-4. Use exactly the rating the manual states. GL-5 carries more extreme-pressure additive, which some OEMs warn against for transmissions with brass synchros.
SAE gear viscosity: Match the specified grade (commonly 75W-90, 80W-90 or 75W-140). Use the heavier grade only when the manual allows it for towing or heavy loads. The wrong viscosity affects shift feel, cold flow and gear protection.
Limited-slip compatibility: Clutch-type limited-slip differentials need a fluid labelled LS or a separate friction modifier, as the OEM specifies. Without the friction modifier, clutch-type limited-slip differentials chatter on turns.
Manual transmission fluid type: Confirm whether the transmission takes gear oil, a dedicated MTF or ATF before buying. Many manual transmissions do not use gear oil at all.
Dual-rated claims: Use a 'GL-4/GL-5' oil in a GL-4 transmission only if the oil is explicitly stated as safe for yellow metals or synchros. Not all GL-5 formulations are safe for brass synchros.

Red Line: Known for MT-90 and similar synthetic manual transmission fluids and gear oils.
Mobil 1 / Mobilube: Synthetic and conventional GL-5 gear oils in common grades.
Valvoline: Widely available conventional and synthetic gear oils.
Lucas Oil: Popular heavy-duty gear oils for trucks and towing.
Motul: Synthetic gear oils with clear GL-4 and GL-5 labelling.
Royal Purple: Synthetic gear oils, including grades for limited-slip differentials.

GL-5 in a transmission that specifies GL-4 only
Non-LS fluid in a clutch-type limited-slip differential without adding the friction modifier
Assuming a manual transmission takes gear oil without checking

Not reliably. Some GL-5 additives can attack brass synchros, so use GL-4 or a fluid explicitly rated safe for both.