This subject comes up often enough I thought I'd re-post this service bulletin and include it in the FAQ.
There's a lot of conflicting info about whether or not to use a posi or limited-slip additive to the gear oil when filling a G80 (Eaton Gov-Loc) differential in a GM truck.
Much of the confusion likely comes from the fact that the G80 RPO code is also used to indicate posi or limited slip differentials in car applications, but in trucks it is the Eaton Gov-Loc. It doesn't help when someone goes to Eaton's site and finds statements like:
"What type of lubricant and limited slip additive should I use with my newly purchased Eaton Limited Slip Differential? Eaton strongly recommends utilizing an API-GL5 approved Mineral-Based 80W-90 Axle Lubricant and one 4-oz bottle of Limited Slip Additive (GM or Ford)."
Note that the Eaton Gov-Loc is NOT a limited slip; the information in the above paragraph does not apply to the Eaton unit used by GM for the trucks!
Here's the service bulletin from GM about the proper lubricant and use of additives (I added the bold and underline):
LOCKING DIFFERENTIAL LUBRICANT (SERVICE INFORMATION) #91-4-109
SUBJECT: LOCKING DIFFERENTIAL (G80) LUBRICANT - (SERVICE INFORMATION)
VEHICLES AFFECTED: ALL LIGHT TRUCKS EQUIPPED WITH G80 REAR AXLE ALL YEARS
Some light duty trucks equipped with locking rear axles (G80) may exhibit rear axle chatter, especially when turning a corner from a stop.
This condition of alternate engagement and disengagement of clutches in differential assembly is usually caused by contaminated axle lubricant.
To correct this condition, drain and refill the rear axle with SAE 80W-90 GL5 (P/N 10950849).
The use of any additive in locking rear axles (G80) is not recommended. Rear axle additives are designed for use in limited slip differentials which are normally installed in cars. All light duty trucks equipped with RPO G80 make use of a locking differential and the use of additives will delay the engagement of the locking mechanism and may decrease axle life.