The Silverado SS is equipped with the Eaton G80 rear differential, a mechanical-locking limited-slip unit. Production tolerances in this axle are very tight, and the clutch pack and seals are specifically designed for use with synthetic gear lube. The axle is filled at the factory with a synthetic GL-5 75W-90 fluid made by Texaco (part #2276) and resold at the GM parts counter as service #12378261 for about $25-30/qt Using mineral based gear lube in this differential will cause errant 'slip-stick' operation in the clutch pack, and if not corrected (by using synthetic fluid) will lead to shuddering around corners and clutch pack failure.
During the first 300-500 miles of life the G80 rear axle develops very high temperatures as the ring and pinion break in (over 350-degrees); also, any dust or casting debris from manufacture will be washed down into the fluid. In these first miles of life avoid towing, high loads, and high-speed extended driving until the ring and pinion have seated and rear axle temps fall to normal. Due to this high initial break-in heat a gear lube change is in order because additives are replenished and contaminants are eliminated (this quoted directly from Eaton). Draining and refilling with synthetic fluid after this initial break-in can be considered 'lube for life' but most conservative mechanics recommend draining and refilling at 40K to 50K intervals, even earlier in dedicated tow/severe duty vehicles.
Failing to drain and refill your rear axle after break-in can result in high noise operation, howling, and/or outright bearing failure as (1) the debris from manufacturing continues to circulate inside the axle housing and tears up bearings and seals, and (2) the lubricity of the factory filled synthetic gear lube is greatly compromised by break-in heat.
Draining and refilling your rear axle gear lube is easy for the novice mechanic, and most non-mechanics can perform this basic service with a little step-by-step guidance. To begin just review this how-to, buy the required gear lube at the local auto parts store, park your truck on a level surface, and start wrenching.
Edited by Mr. P., 18 August 2008 - 01:22 PM.