Historically when I have changed diff fluid use the 80w-90 with the limited slip additive.

Today at the GM dealer I picked up some oil for the rear end and the shop manager told me I don't have to use the limited slip additive with their newer oils. I asked a couple of time is he sure???

The oil I bought was AC DELCO 1 #89021672 which is AXLE LUBRICANT SAE 80W-90 indicates it is compatible with limited slip friction modifiers. The guy said that this means that this oil already has the modifier/additive in the oil. To me this really means you can use the additive with this oil without problems hence the word compatible.

I bought the limited slip additive anyways as this doesn't seem right. Does anyone know for sure could it be that their new oils don't need additive???

I am in Canada not U.S. so part numbers may not work???

 

This is from GM's own service information:

"Lubricant Level Check
Raise and suitably support the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
Clean any dirt or foreign material from around the rear axle housing drain plug.
Remove the rear axle housing drain plug.
Verify that the axle lubricant level is within the following range:
No higher than the bottom of the plug opening
No lower than 15 mm (9/16 in) below the plug opening
If necessary, add to or replace the axle lubricant. Use synthetic 75W-90 gear lubricant (GM P/N 12378261), meeting GM specification 9986115, or equivalent.
If the vehicle has a limited slip differential, add 118 ml (4 fl oz) of limited slip differential additive (GM P/N 1052358), or equivalent.

Notice
Use the correct fastener in the correct location. Replacement fasteners must be the correct part number for that application. Fasteners requiring replacement or fasteners requiring the use of thread locking compound or sealant are identified in the service procedure. Do not use paints, lubricants, or corrosion inhibitors on fasteners or fastener joint surfaces unless specified. These coatings affect fastener torque and joint clamping force and may damage the fastener. Use the correct tightening sequence and specifications when installing fasteners in order to avoid damage to parts and systems.


Install the rear axle housing drain plug. Tighten
Tighten the rear axle housing drain plug to 35 N·m (26 lb ft).

Lower the vehicle.

Document ID# 334589
2000 Chevrolet/Geo Camaro "


I'd say use the additive.

This is for the new GTO with the latest info:


Lubricant Change
Raise and support the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
Place a drain pan or suitable container under the rear axle housing.



Remove the rear axle housing oil level fill plug (1) from the rear axle housing.
Remove the rear axle housing drain plug (4) from the rear axle housing and drain the rear axle housing.
Notice
Refer to Fastener Notice in Cautions and Notices.

After the rear axle housing has completely drained, install the rear axle housing drain plug (4) into the rear axle housing. Tighten
Tighten the rear axle housing oil level filler plug to 27 N·m (20 lb ft).

Refill the rear axle housing with approximately 1.0 liter (1.0 qt) of Synthetic Gear Oil 75W-140 GM P/N 89021809.
Using a graduated measuring cup, add 25 ml (1.0 oz) of Limited Slip Differential Friction Modifier GM P/N 89021958 to the rear axle housing.
Fill the rear axle housing with the remaining 0.575 liter (0.644 qt) of Synthetic Gear Oil 75W-140 GM P/N 89021809. The oil level should be even with the bottom of the rear axle housing oil level filler plug hole.
Install the rear axle housing oil level fill plug (1) with a new gasket and the lubricant tag (2) into the rear axle housing. Tighten
Tighten the rear axle housing oil level fill plug to 27 N·m (20 lb ft).

Remove the drain pan from under the rear axle housing.
Lower the vehicle.

Document ID# 1211330
2004 Pontiac GTO

 

thanks

 

If you have a torsen you dont need the modifiers though.. you only need the modifiers with the cone or clutch style LSD's. Also if you get royal purple or redline, then you dont need them because they are pre mixed in.