New Ring and Pinion Break In Procedure Because of the slight roughness of the new gear contact faces and the heavy preloads that must be applied to the new tapered roller bearings, your new rear axle will generate its maximum amount of waste heat in its first few miles. With use, bearing preloads diminish and gear surfaces become smoother. Therefore, the first few hundred miles on a new gear set are the most critical. To prevent premature (and almost immediate) failure of your new gear set, this break-in procedure must be followed: All new gear sets require a break in period to prevent damage from overheating the gear oil. The lubrication properties of overloaded or overheated gear oil will be diminished. If the gear oil to breaks down, the end result will be catastrophic damage to the ring and pinion. Avoid burn outs, sudden starts and heavy acceleration for the first 500 miles. Drive the vehicle gently for 15 – 20 miles, but no longer than 25 - 30 minutes to build up the heat. Do not exceed 50 mph. Let the differential cool down for one hour. Before driving again, feel the differential with the back of your hand. The rear differential should be cool to the touch. Repeat this cycle five times. When driving on the highway, vary your speed while avoiding heavy acceleration. I cannot overemphasize the importance of changing the gear oil after the first 500 miles. This will remove all metal particles, black phosphoric coating and other contaminants shed by the gears during the break in period. The integrity of the gear oil is compromised as a result. The gear oil must be changed. When a posi unit is rebuilt along with a gear change, the clutch discs will break in and settle as well. There will be additional metal particulate in the gear oil from the clutch discs. Again, it is extremely important to change the gear oil after the first 500 miles. I always recommend a good quality petroleum based 80W90 gear oil with GM Limited Slip Additive. DO NOT use synthetic lubricants in clutch type limited slip differentials. Synthetic oil makes the clutch discs too slippery. This will not allow the clutch discs to lock up. When the clutch discs slip, one wheel spins. This defeats the purpose of using a Limited Slip differential. Using synthetic gear oil will eventually render the limited slip operation useless.