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Gear oil keeps components such as the ring and pinion gear set, spider and side gears, limited slip differentials, axles, bearings, and the like lubricated and running smooth. Friction modifiers are additives that improve the performance and efficiency of clutch style limited slip differentials. Understanding how these vital products work is key to properly maintaining your driveline so it performs in the harshest of conditions and lasts for years to come.
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. Take a multi-grade oil like an 80W90… The 80W is the viscosity value in the winter, denoted by the W, or more precisely at 0 degrees F. So, an 80W oil is more viscous than a 60W oil at 0 degrees. The second number, 90, refers to the oil’s viscosity rating at 212 degrees F, again a higher number will be more viscous at operating temperature. These numbers are also a function of thickness. So, an 80W oil is thicker than a 60W oil. Multi-grade oils are blended with polymers that slow the rate of thinning as temperatures rise while also slowing the rate of thickening as the mercury falls, determining the viscosity range of the oil.
Generally speaking a thinner oil will circulate faster through the differential (or engine for motor oil) during a cold start. Conversely, a thicker oil will protect better when the diff gears generate higher temperatures.
The "GL-5" designation identifies how much Sulphur-based friction modifier is present in the oil. This chemical modifier helps the oil function under extreme pressure… like in an LSD. Note: Because of its Sulfur content GL-5 oil is meant to be used on hypoid gears in differentials only; it is too corrosive to be used in transmissions, which require a milder GL-4 formulation because they contain soft metal parts.
Yukon offers a number of gear oils in a variety of viscosities and in synthetic and non-synthetic blends.
Stand-Alone Friction Modifiers
Yukon Friction Modifier Additives are used when more clamping force is needed. This type of product is for clutch-type differentials 
like the Yukon Dura Grip only, not gear-driven limited slips like our Spartan Helical LSD. Friction modifiers are all about annoying chatter. Yukon Friction Modifier Additive guards against abnormal clutch engagement and/or disengagement (source of the chatter), ensuring the optimal friction characteristics between clutch discs are maintained. These additives can be used to compensate for worn clutches or to “tune” the performance of the clutch packs for a specific application like road racing or vehicles that see a lot of track days. It should be noted that is our own proprietary formula designed specifically for the composite clutches in Yukon LSDs. If you run a Dura Grip, do not use parts store grade modifiers because they are formulated for OEM paper-style clutches. With about one-fifth the abrasive properties these store-bought products will perform like watered down versions of our stuff.
These products are vital to the proper upkeep of your differential and knowing how they’re rated and how they do their job is critical to making informed maintenance decisions for your vehicle.
