I just bought a 2001 Silverado 2500 HD with a 8.1 L Vortex and Allison 4 speed auto trans. The RPO codes (GT4 & G80) indicated it has a 3.73. positraction, limited slip rear end. It has a 14 bolts and is 13 inches wide. The gear oil is dirty and needs changed. I find nothing in the owners manual as to what oil to use.

Mitchells1 online repair manual (web site) says use SAE 75E-90 Synthetic "IF" there is a tag on the differentials stating synthetic required. But it cautions to use synthetic only if required, and to not use a friction modifier with a limited slip or locking differential. Other forums that I've visited say friction modifiers are needed for limited slip rear ends. The Mitchell site indicates that 8-1/2, 8-5/8 and 9-1/2 ring rears were used for K series. Which doesn't seem completly correct, because, Drivetrain.com has an rear end identifier (based on bolt pattern and width) which suggests I have a 10.5 ring gear, but also states that they were not used on 2001s. It states that 11.5s were used on 2001 and up (But doesn't show the 11.5 on the identifier??? Drivetrain.com indicates that an iditification number should be on the frount of the axle tube on the right that will define the ratio, manuf. info and date made.

I find no tag (per Mitchell) or ID number (per drivetrains). There is a casting number (40006189 F) on the bottom side of the left (driver side) web in frount of the axel tube. I have surched in vain for the significance of this number.

I'm thoroughly confused!!

 

id repost this at performancetrucks.net, people on that forum really know what they are talking about