What is a typical rear axle lubricant specification?

• A. SAE 80W-90
• B. GL5 85W-90
• C. GL4 75W-90
• D. GL5 85W-90

Answer: (D)

Rear axles deal with very high loads on gears that slide against each other, so the lubricant needs strong protection and a viscosity that stays thick enough under heat. The GL-5 rating signifies an extreme-pressure additive package designed for hypoid gear sets, like those in rear differentials, while 85W-90 is a heavier, durable viscosity that maintains a protective film at operating temperature. That combination—EP protection plus a robust viscosity—is why GL5 85W-90 is the typical rear axle lubricant specification. Lighter or non-EP options, such as an 80W-90 without GL ratings, or a GL4 with 75W-90, don’t provide the same level of protection for these gears, making them less common choices for rear differentials.