Gear Oil & Friction Modifiers: What They Do and Why They Matter

Gear oil keeps key drivetrain components—like the ring and pinion gear set, spider and side gears, limited slip differentials, axles, and bearings—lubricated and running smoothly. Friction modifiers are additives that enhance the performance and efficiency of clutch-style limited slip differentials. Understanding how these vital fluids and additives work is essential for maintaining your driveline’s performance, longevity, and reliability under the harshest conditions.

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Differential Oil by the Numbers

Yukon recommends non-synthetic 80W-90 GL-5 gear oil in all differentials. Oil is rated by its viscosity value, which represents how thick or thin the fluid is at certain temperatures. Take a multi-grade oil like 80W-90:

80W – the viscosity at 0°F (the “W” stands for winter). An 80W oil is thicker than a 60W oil at that low temperature.
90 – the viscosity at 212°F, the oil’s behavior at operating temperature. A higher number means a thicker oil under heat.

Multi-grade oils are blended with polymers that reduce the rate of thinning as temperatures rise and slow thickening as temperatures drop. The result is a product that performs well across a wide temperature range.

Generally, a thinner oil circulates faster through the differential during cold starts, while a thicker oil offers better protection when gears generate higher temperatures under heavy load.

Understanding the GL-5 Rating

The GL-5 designation identifies how much sulfur-based friction modifier is present in the oil. This chemical additive helps the oil maintain film strength and protection under extreme pressure—perfect for hypoid gears in differentials and limited slips.

Important note: Because GL-5 oils contain high sulfur levels, they are not suitable for manual transmissions. The sulfur can corrode soft metal parts like brass synchros. For transmissions, a milder GL-4 formulation is required.

Yukon offers a full range of gear oils in various viscosities and both synthetic and non-synthetic blends to meet every performance and climate need.

Stand-Alone Friction Modifiers

Yukon Friction Modifier Additives are used when additional clamping force is needed in clutch-style limited slip differentials, such as the Yukon Dura Grip. These additives are not intended for gear-driven LSDs like the Spartan Helical LSD.

Friction modifiers help eliminate annoying clutch chatter by ensuring smooth engagement and disengagement of the clutch packs. Yukon’s proprietary formula is designed specifically for the composite clutch discs used in Yukon LSDs. It delivers the optimal friction characteristics for consistent performance and longevity.

Our friction modifier can also help compensate for slightly worn clutches or fine-tune the behavior of a differential for specialized uses—like road racing or vehicles that see frequent track days.

Note: Avoid using parts-store-grade modifiers in your Dura Grip. Those products are formulated for OEM paper-style clutches and lack the proper abrasiveness for Yukon’s composite design. They perform more like diluted versions of our advanced formula.

Maintenance Matters

Both gear oil and friction modifiers play a critical role in differential health. Knowing how these products are rated and how they perform helps you make informed maintenance decisions that keep your Yukon-equipped driveline performing its best for years to come.

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