rear diff lube

tinman · April 14, 2002

I am getting ready to change to rear diff oil in my two trucks. I have heard that I should use the gm synthetic. I have also heard that amsoil is great too. One truck has a posi and the other has a open rear end. Any advise you could give would be great :ehh:

SoMnDMAX · April 15, 2002

GM can stick their "required" oil as far as I'm concerned. :angry:

I use Mobil 1 75w-90 synthetic gear oil in all my vehicles, whether it's my 2500 HD or my 10 second drag truck.

Amsoil also makes a very good oil, and Redline makes a good one as well. I wouldn't be afraid to run any of the three.

HillHound · April 15, 2002

I am getting ready to change to rear diff oil in my two trucks. I have heard that I should use the gm synthetic. I have also heard that amsoil is great too. One truck has a posi and the other has a open rear end. Any advise you could give would be great :ehh:

I bit the bullet and bought the overpriced synthetic GM oil when I changed mine because of the posi axle (locking diff). Figured since I don't plan on changing it again anytime soon (if ever) I'd just go ahead and use it. I drained it after about 10000 miles and some light towing, removed the cover and cleaned the magnet then cleaned off the gears with brake fluid.

If you have an open diff I don't think it's going to matter what you use. For the posi diff my opinion is to replace it with GM's oil. It is overpriced but if it ensures the proper working of the differential for the next 75000mi or more I'll spend the extra $50 or so and use it. Like I said though, that's just my opinion and it probably just comes down to personal preferance.

carguru · April 15, 2002

I am getting ready to change to rear diff oil in my two trucks. I have heard that I should use the gm synthetic. I have also heard that amsoil is great too. One truck has a posi and the other has a open rear end. Any advise you could give would be great :ehh:

I bit the bullet and bought the overpriced synthetic GM oil when I changed mine because of the posi axle (locking diff). Figured since I don't plan on changing it again anytime soon (if ever) I'd just go ahead and use it. I drained it after about 10000 miles and some light towing, removed the cover and cleaned the magnet then cleaned off the gears with brake fluid.

If you have an open diff I don't think it's going to matter what you use. For the posi diff my opinion is to replace it with GM's oil. It is overpriced but if it ensures the proper working of the differential for the next 75000mi or more I'll spend the extra $50 or so and use it. Like I said though, that's just my opinion and it probably just comes down to personal preferance.

I second that. The open diff doesn't matter as much, but that locker is one expensive beast if you have to replace it down the road. I would change the diff fluids especially if you launch a boat and get the axle under water.

westex39 · April 28, 2002

Amsoil series 2000, 75W-90.

99silveradoz71 · April 29, 2002

Anyone have pictures of the differential change or instructions? I'm not particularly mechanically inclined, but willing to perhaps give some of the routine stuff a try. I do my oil changes from time to time with no problems. Any help/info's appreciated. Thanks.

MountaineerTom · April 29, 2002

Here you go. Front/Rear diff fluid change I cna't remember who's page this is. ???

And here's some info from my web page. Differential

gandolphxx · April 29, 2002

I am running Redline 75/90 - quite happy with it.

CMNTMXR81 · April 29, 2002

I use the same stuff Hillhound uses and I have yet to grenade my little 7.5" gear in the cheap 10 bolt carrier. I know it's just a matter of time, but something must be going right back there for it to hold on soo far.

MarkD · April 29, 2002

Amsoil 75W-140

TXDMAX · April 30, 2002

I used Pennzoil 75w 90. I checked and it does meet the Gm specifications along w/ the other fluids listed here. They are all good fluids just make sure it is GL-5 rated.

TxDoc · April 30, 2002

Tinman-when you say "posi", I did not know if you meant the limited-slip or mechanical locker.

Just some other info from and earlier discussion (thanks to DmaxMaverick):

"The Eaton limited slip rear end does require mineral based oil and additive. Synthetic is too slippery and the clutches don't get enough friction to engage and balance torque."

"The [Eaton] G80 is not a limited slip rear end, by traditional terms. It is referred to as the "Gov-Loc" rear end. It does have carbon clutches, but they are engaged by a governor. Using mineral based oil and an additive in this rear end will very likely greatly reduce its useful life. The use of only mineral based oil, without an additive, wouldn't be nearly as bad. The design did not include the use of the additive, which, among other things, contains mostly acid. The acid prevents the oil in traditional limited slip rear ends from glazing to the clutch surfaces, which causes the chatter by preventing lubrication. Synthetic oil will not glaze to the clutches like mineral based oil, and over lubricates the clutches. The G80 does not maintain constant friction like the older limited slip models. It only engages the friction clutches when there is a given amount of differential wheel speed. This has many advantages. Namely the reduction of heat in the rear end, and greatly reduced wear of the friction clutches, as they are not under constant load. See for yourself. Jack up the rear end, both wheels. It will act like an open diff. when one wheel is spinned. Notice no felt friction (or there shouldn't be, anyway). Additionally, synthetic oil should be used in the front end of a 4 x 4. Although it is not under the same load, or used as often as the rear end, they are usually neglected by rare service. Mineral based oil will absorb water (from submersion or condensation) and decompose over time (Yes, mineral oils decompose, after all, it is organic. That is why old oil gets that rank smell). Synthetic oils have an indefinite static life. Cheers (Boy! This should stir the pot, not that it is intended to)."

Eaton limited-slip:

http://www.torquecontrol.eaton.com/prod3.htm

Eaton mechanical locker:

http://www.torquecontrol.eaton.com/prod2.htm

Eaton FAQ:

http://www.torquecontrol.eaton.com/prod2.htm

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).

Like I said, some people call a posi a mechanical locker and some a limited slip. So, the info may help you or at least get you to the maker for more detailed info. Hope this did not confuse things.

Pancho · May 1, 2002

I have not had the time to follow up on the links provided in the last post but is there any problem using Mobile 1 75w-90 synthetic in both the front and rear differentials? The 2500HD owners manual calls for 80w-90 in the front and 75w-90 in the back.

SilveradoWV · May 1, 2002

I have noticed that sometimes GM will say to use GM part # whatever or Equivalent. So either way you go it will probably end up working out good. But if it came with synthetic I would put synthetic back in it. Just my .02

tinman · May 2, 2002

When I said I have a posi rear end , I ment what ever the truck came with stock. i assume the G80. The term posi meant to me any type of locking or limited slip rear end. So if I have the G80 rear end I must use a synthetic oil. I will problely go with the gm stuff. I found it online for 19 bucks a quart at Scoggin-Dickey parts. That much better than the 27 bucks a quart that gm whats for there stuff.