What type of rear in oil should I use?

[sewlow]

Additives are to prevent & reduce the clutches in a Limited Slip Differential from chattering. Gov-Bombs don't have clutches.

LSD-equipped diffs will have this tag.

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An LSD. ...or what is more commonly referred to as a 'Posi'.
Plates, clutches & springs. What to look for if the tag's missing.
A Gov-bomb looks like a cuckoo clock in comparison.

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[someotherguy]

Seeing as the thread is reactivated.....
The additives being mentioned - what are they, what is their function?

A GL5 will already have a fair amount of anti-wear additive. Anti-wear additive of the type that attacks 'yellow' metals which is why GL5 can only be recoomended in gearboxes with steel synchro rings and anything with brass/bronze synchro rings should use GL4.

The additional additives (diff) are they anti-wear?

And, seeing as diffs are mentioned by size - what will my 1999 C2500 with G80 and 4.1 ratio have?

sewlow addresses the additives pretty well in his response... but yeah! That's something that drove me nuts about my old Unimog; I had to buy two different gear oils for it. GL-4 for the transmission and transfer case, and GL-5 for the axles.

Your 2500 has the 14 bolt rear, either 9.5" (semi-float) or 10.5" (full float) - easy to tell apart by a protruding hub with bolt-in axles on the full floater. 2500's came with both; typically the later 400's came full float, at least in the pickups. 3500's are full float only.

Additives are to prevent & reduce the clutches in a Limited Slip Differential from chattering. Gov-Bombs don't have clutches.

LSD-equipped diffs will have this tag.

You must be registered for see images attach

You must be registered for see images attach

An LSD. ...or what is more commonly referred to as a 'Posi'.
Plates, clutches & springs. What to look for if the tag's missing.
A Gov-bomb looks like a cuckoo clock in comparison.

You must be registered for see images attach

Yep and bears repeating for anyone in here that didn't already know, GMT400 trucks never came from factory with a posi unit. Only open differentials, and the G80 Gov-Loc. They not only look like a cuckoo clock, they kinda act like 'em too..

Richard

 

[Schurkey]

WTF do you think a GM dealer parts counter is going to do? They're going to look up your vehicle, by VIN, and give you the GM recommended gear oil for the specific rear end installed in your vehicle.

As for synthetic, what are you, one of those "all oil is the same" Luddites? Or did you just not know that synthetics weren't in wide use other than in the Corvette two decades ago when that truck was produced? Just because it wasn't factory then doesn't mean you shouldn't or can't use it. They didn't sell the trucks with premium tires either, do you tell people that putting better tires on their truck is stupid? Or should they only use all season Uniroyals made in 1999?

The fact is that synthetic gear oil is proven better in hot, cold, towing, and performance conditions. Period. And considering how seldom you actually have to change it, it's stupid not to upgrade to it.

GM has a bulletin about NOT using synthetic in any 9.5 axle produced before a certain date, and not using it in 10.5s produced before a certain date unless the RTV silicone hub seal is replaced with a mechanical gasket. Conventional "Dino" grease only.

So, no, synthetic is not "proven better", "period".

Personally, I'm not entirely convinced...but GM's engineering department is.

Additives are to prevent & reduce the clutches in a Limited Slip Differential from chattering. Gov-Bombs don't have clutches.

Of course Gov-Locks have clutches.

 

[1997]

i've used synthetic in my 95 pretty much from day 1. no leaks.
hub seal is that the axle seal?
I used a gasket on the cover since i've RTV challenged anyways.

 

[Frank Enstein]

Redline heavy weight shockproof is my favorite for hard working axles. It does have friction modifier it though.
The Summit part number is RED-58204. $20 a quart but it's really good stuff.

 

[Pinger]

Looks as though 3 quarts is what you should get

For the Rear Differential Capacity.....

Rear Drive Axle (8.6") 2.03 liters (2.15 quarts)

Rear Drive Axle (9.5") 2.60 liters (2.75 quarts)

Rear Drive Axle (9.75") 2.84 liters (3.00 quarts)

Rear Drive Axle (10.5") 2.60 liters (2.75 quarts)

Rear Drive Axle (11.5") 3.00 liters (3.17 quarts)

I keep seeing 'xx bolt' being mentioned in relation to diffs. Is this the bolt count holding the rear plate?
If so, my C2500 has a 14 bolt diff/axle. Which of the above is it?

 

[Schurkey]

Yes, that's the number of bolts holding the rear cover. On some designs, it's also the number of bolts holding the ring gear. There are two common "14 bolt" axles in these trucks; one has a 9.5" ring gear, and semi-float axle shafts. The other has a 10.5" ring gear, and full-float axle shafts. Either one could be in a 3/4-ton truck. The break-point seems to be 8600 GVW; the lighter 3/4-tons get the 9.5 while the heavier trucks get the 10.5. If the rear axle by the lug nuts looks like a passenger car, you've got the 9.5 semi-float. If the rear axle by the lug nuts has a bigass hub sticking out, the size of your fist you've got the 10.5 full-float. Both "were" Corporate GM designs, but the axle manufacturing got sold-off. The "new" company name is American Axle and Manufacturing (AAM).

 

[Pinger]

Mine has 8600lb GVW. I'll have a look tomorrow for the 'hub' (visible behind the brake back plates presumably?). No sign of any tag indicating 'LSD lube only' - build sheet says G80. My handbook says 80W-90 oil. By luck, I have just the oil for it.

 

[454cid]

Mine has 8600lb GVW. I'll have a look tomorrow for the 'hub' (visible behind the brake back plates presumably?).

No..... just walk up to the truck and look. It's RIGHT there, (edit: under the center cap.) Full floating and semi-floating isn't a GM thing. It's universal to powered solid axles.

No sign of any tag indicating 'LSD lube only' - build sheet says G80. My handbook says 80W-90 oil. By luck, I have just the oil for it.

LSD lube only would be a car thing.... GM used limited slips in cars. Lockers were put in trucks, and they don't use clutches the way a limited slip does. You don't want any LSD additive in with the locker, as it will make it misbehave.

Edit: You have a 9.5" axle

 

[Schurkey]

Yes, the truck "G80" "Gov-Lock" does NOT use "posi additive".

Yes, the truck "G80" "Gov-Lock" does have clutches. They're just operated differently than in a car. I've heard they're "carbon-fiber" clutches, and don't react well to "posi additive".