Changing axle oil 2003 GMC K2500HD

Getting ready to change the rear axle/differential oil in my 2003 GMC K2500HD truck, and I have a few questions.

Basically looking for oil brand recommendations. Manual says to use synthetic 75w90 GM P/N 12378261/ specification 9986115. Checked the local O'Reilly and the guy at the counter looked up my truck and GM P/N 88900401 is showing as a replacement for the numbers in my manual. Options available were the O'Reilly MasterPro, Royal Purple, Castrol, and Lucas Oil. I am leaning towards Lucas or Royal Purple, have heard good things about both. Any thoughts or experience on what brand to choose?

Other question is about having or using a limited slip additive. Truck has RPO code 'G80' which is for the locking differential. A bit confused as I see conflicting opinions on use of limited slip additive. Some say yes and others no. A search on the net turned up a GM Service Bulletin saying not to use a limited slip additive in the truck axles with RPO G80. I looked up the specs on the Royal Purple and it has the limited slip additive, and the Lucas oil does not. Not interested in Castrol, and the counter guy basically told me not to buy the MasterPro if I planned on keeping my truck for a while.

So it looks like Lucas Oil is the product I am looking for. Or is there another source/brand? I can go to the GM dealer and buy there. Specs I have show the differential holds 2.75 Quarts of fluid. Going to replace the cover gasket as well.

Any help would be appreciated.

I use synthetic 75w140 at my shop in all
rear ends, for changing or on rebuilds. I
feel it holds up better to the heat of
towing. Synthetic 75w140 is OE on Fords.
Brand is not important, but Valvoline is
most accessible to me at a decent price.

It can be hard to find but I used 85W-140 Amsoil in the rear axle of my '96 F-250. Supposedly it has/had
the friction modifier additive in it but I added the recommended two bottles anyhow. 300,000 miles and
two oil changes and the Traction-Loc still works like new. Ford released a TSB changing ALL drive axles
to synthetic 85W-140 oil around 1999-2000.

Good luck finding the cover gasket. Not sure about GM but Ford hasn't used a cover gasket in 30+ years,
maybe more like 40, since the started using the 8.8 and o.75 axles. RTV only used, I recommend The RIGHT
STUFF.

Whut Sprint 6 said plus a little Wynns Friction Modifier is not gonna hurt it... When in doubt I add a tube of it even on a older standard transmission/differential car, truck , motorcycle are tractor its served me well...

I would order either one of the GM part #s online. Price is $15.55 per quart. You know it works. Dealer list is about twice as much.

The G80 Eaton locker ( aka governor locker or govbomb) does not need additives, just plain oil. That GM service bulletin you read is true and has been around a long time for the G80's. , This has a locking diff, not a limited slip, hence no additives needed.

I wouldn't worry about master pro oil being inferior, its not a bad product, who knows it could be better that some higher end brands. For 1-2 dollars more per quart you can probably feel a little more confident on the "better" brands though. Lucas, Valvoline, Castro are all good brands. If this truck see's a lot of heavy towing or extreme temps cold or hot, I would always run synthetic and probably consider 75w140 over the spec'd 75w90.

This axle should have a drain plug, but if you want to remove the cover, go ahead. I'm not sure if you will find a gasket, I don't know of anyone that uses them. I would use permatex ultra black, an excellent gasket maker specifically for drivetrain.

There are probably a million G80's that some uniformed person put friction modifier in. A lot of these additives are weak to mild anyway, but if these additives are going to do anything to a G80 locker it will hurt them, maybe not so much physically but they will get delayed engagement when locking.

I had a super hard time getting the plug out of my 2000 Chevy 2500. Air chisel finally did it. Took me couple hours using all the tricks I knew. Could've sworn it was welded in there.

Use a cover gasket if you pull the cover,
Felpro makes them for both all units you
could have, 9.5, 10.5, or 11.5. Do not use
any type of silicone, synthetic gear oil
eats silicone sealer. All OE's have
switched back to gaskets for this reason.
When I get a truck in with silicone on the
cover, it's always leaking all over.

If that type of posi has clutches in it, it will need the climbing additive. No clutches, no additive needed. Gear grease with the additive won't hurt anything, you just don't need it. My 05 Dodge with the 11.5 is considered "limited slip", and does not have clutches, so I use a synthetic like Mobil 1 or Red Line without additive.

I have never had one leak that I sealed with RTV that being said I changed to RTV for gear lube to be safe... They either used a cheap RTV are not one for the application are did not clean it proper...

On all I like for it to take a good set before I add the oil :wink:

Sprint 6,

Will the 75w140 be a problem in cold climates? Winter around hear sometimes get -20F or more (no heated garage or shop, truck sits outside), would the heavier oil work ok? Temps are going to be upper 90's for the next week. Temp extremes for this area common during the year.

Valvoline is my go to brand usually, but that brand is special order in this case.

4play,

Truck is mostly a daily driver, not a lot of towing or heavy loads. Trailers and towing are always a possibility around the farm , so keeping my options open.

2x4,

Not looking forward to getting the plug out of the differential. Probably going to keep the torch nearby just in case. Did a brake job on the truck a few months back, broke a 1/2" drive socket and a 3/4" drive socket getting one of the front caliper/bracket bolts out. Ouch!

Thanks to all who replied, the information is much appreciated.

Should have added a little more to my original topic, the reason I am changing the oil is the axle vent hose is damaged and has holes in it. Not sure if this was normal wear and tear, or if a rodent chewed on it. Either way, between the heavy rains and water across roads recently, and the deep snow drifts I went through the past winter, probably best to replace axle vent hose and change oil as a precaution.

Anyone have a preferred source for the axle vent hose? Not finding any in parts look ups, thinking maybe common rubber hose from auto parts store?

Truck has 93K on it, so hoping to get many more miles out of it.

I guess I should look a little closer, I don't recall seeing a drain plug on the axle? 10.5" GM 14 bolt, IIRC has a check/fill plug on the passenger's side of axle by the driveshaft. It's been a while since I changed the oil in one of these axles, and that was a much older model. I just assumed that I had to pull the cover to drain the oil, and would need a new gasket to reinstall.

Looks like the cover gasket is available for less than $5. Definitely worth that to prevent leaking fluid, this stuff is not cheap.

When I changed the differential on my 06 Yukon XL, no drain plug and the cover gasket was a reusable type.

Well, thinking about it, there are many variations/generations of the 14 bolt, some had drain plugs, some didn't. It might be the difference in having the larger ring gear ( 11.5 ) or newer/older model axle. Anyway it's no big deal to drain it from the cover, just a little more hassle and time.

To consider the oil in cold climate use, a 75w90 or 75w140 are both 75w oil when cold. When the oil gets hot, the 75w140 will be a little thicker than the 75w90.

A lot of the diff. fluids around here the limited slip additive in the fluid regardless. Whatever you choose I would not use a gasket. As HOBO said use a gear lube rated RTV. RTV was made to be used between a stamped cover and a housing.

Cold shouldn't be a problem for the 75w140,
the 75w is the same as the 75w90, so it
won't be a whole lot thicker cold.

My 2001 gm 14 bolt has a drain plug no need for gasket.