Which lubrication specification is typical for a rear axle?

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

Answer: (B)

Rear axles with hypoid gears endure high sliding contact and heavy loads, so they need gear oil with extreme-pressure additives to prevent wear. Oils that carry a GL-5 rating are designed for this service because the EP additives form a protective film under high pressure. The 85W-90 viscosity provides a strong lubricating film at load while still flowing adequately when starting up and as temperatures rise. This combination—GL-5 with 85W-90—is commonly specified for rear axles, giving reliable protection across a wide range of operating conditions.

An 80W-90 oil may work in some cases, but without the GL-5 specification it doesn’t guarantee the necessary EP performance for hypoid gears. A 75W-90 oil is lighter and often seen in manual transmissions or certain axles, but it isn’t as universally suitable for typical rear-axle service. SAE 90 is a straight-weight oil with limited temperature performance and no guaranteed EP package, making it less appropriate for modern rear axles that rely on a robust protective film.