API GL-4 vs GL-5 Gear Oil: Key Differences & Which Your Transmission Needs
API GL-4 is a mild extreme-pressure gear oil for manual transmissions and transaxles with yellow-metal synchronizers. API GL-5 is a much stronger extreme-pressure formulation built for hypoid differentials and axles; its extra sulfur-phosphorus additives can corrode brass if used where GL-4 belongs.
Drivers confuse them because both say “75W-90” on the bottle and “API GL” on the label. A shop hands you “gear oil,” you top up at the parts store, or you see “GL-5 is newer so it must be better,” and suddenly a Honda Civic gets differential juice in its gearbox.
Key Differences
GL-4 treats gears gently and keeps synchronizer sleeves alive. GL-5 adds up to twice the EP chemistry to stop axle scuffing, but that same chemistry attacks copper and bronze. If the spec sheet calls for GL-4, GL-5 voids the warranty and ruins the syncros.
Which One Should You Choose?
Check the fill cap or owner’s manual: if it says “GL-4,” use GL-4. If it says “GL-5” or “limited-slip differential,” use GL-5. Never upgrade “just because.” When in doubt, call the dealer with your VIN—they’ll read the exact spec in ten seconds.
Examples and Daily Life
Your 2015 Mazda 3 manual takes GL-4 75W-80. Your Jeep Wrangler rear axle takes GL-5 75W-90. A Toyota Tacoma 6-speed manual specifies GL-4 in the gearbox and GL-5 in the diffs—two separate bottles for one truck.
Can I top up GL-4 with GL-5 in an emergency?
Short distances, maybe; prolonged use eats synchronizers. Flush and refill with the correct spec as soon as possible.
Does synthetic change the rule?
No. Synthetic base oil doesn’t override the GL-4/GL-5 additive package; follow the spec, not the marketing.
What if the bottle says “GL-4/GL-5 compatible”?
That means the oil meets both minimums. Verify the manufacturer approves it for your specific transmission model before trusting the dual label.