I like to use synthetic 75w140 in my rear diffs. If it's not an open diff then I add limited slip additive.
Be careful...
Limited slip additive is for Positrac diffs like you find in cars. It won't do a thing for a regular open diff.
If you have a truck with the G80 Gov Loc option this will get you in real trouble. GM released a TSB specifically stating that you do not use limited slip additive with the G80 optioned axles and why not to.
Iirc, most of the gear lube on the market nowdays has the modifiers already added.
That's why you carefully read the bottle... and be informed about what you need.
The below referenced GM
10950849 fluid is 1 Litre of GL-5 80W90 axle gear oil. Any GL-5 80W90 should do as long as it doesn't state it's for limited slip axles...
If it's a plain vanilla open diff you can use any GL-5 gear oil of the correct weight even if it does have the limited slip additive.
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GM Service Bulletin 91-4-109
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. The use of additives will delay the engagement of the locking mechanism and may decrease axle life.
VEHICLES/COMPONENTS INVOLVED:
Some light duty trucks equipped with locking rear axles,
RPO G80. SERVICE PARTS INFORMATION:
Part Number Description 10950849 Lubricant, Rear Axle (1 litre). Parts are currently available through CANSPO.
WARRANTY INFORMATION:
As specified in Light Duty Truck Maintenance Schedules, locking rear axle fluid drain and refill is required owner maintenance at the first engine oil change. Failure to drain and refill the rear axle as specified may contribute to a later axle chatter condition. Refer to the appropriate Light Duty Truck Maintenance Schedule or service manual, section OB, for further details on change intervals.
