We get asked this question almost every day, and the answer is straight up: you should only run 85W-140, API GL-5-rated conventional (non-synthetic) gear oil in a Currie axle assembly. You may hear about how synthetic gear oil provides reduced friction and wear; however, it may not be optimal for your setup – especially if you have a Ford 9-inch rearend. Hear us out.
The 9-Inch Gear Is Unique
The design of the 9-inch features a high offset that places the center line of the pinion gear 2.25 inches below the center line of the ring gear, which is substantial compared to the 1.125 to 1.5-inch offsets of other gear sets (see image below). The high offset distance increases the contact patch between the ring and pinion teeth by approximately 30 percent and is a key reason for the legendary strength of the 9-inch gears. However, strength comes at a price, as this high offset distance increases sliding friction and heat across the gear tooth face. Because of this, not using high-shear oil such as Currie’s 85/140w can lead to scoring and certain gear death, especially during extreme scenarios such as off-roading and racing. The increased pressure capabilities and wear resistance found in Currie’s GL-6 rated gear oils are designed with all this in mind to ensure your rearend’s gears and bearings are adequately protected under the most severe conditions.
Pinion offset distance compared
Conventional Oil Is Proven
Because many of today's commercially available synthetics are not engineered with the 9-inch's requirements in mind, Currie advises using only conventional 85W-140 non-synthetic GL-5 rated gear oil in all front and rearend applications. The viscous (thick) base stock of the 85W-140 oil (check out the video) provides a cushion that helps protect the gears from shock loads and clings more effectively, pulling heat away from the tooth face and preventing localized heat buildup.
