Differential oil: to use synthetic or not?

[bdv]

Dear Dr. Lee

I have a 94 Suburban K2500 4x4 with a 6.5 L Turbo Diesel. It has a 4.10 rear differential. At this time I have the front differential off to replace leaking seals. I have posted topic on which oil I should use when I put the differential back. The responses have said to go with a synthetic oil (I am planning to use Valvoline 75W-90) if it is not a limited slip, and if it is to use regular oil with a GM additive. My question is why not synthetic all around and why an additive? Also I have purchased Suburban used, how do I tell if front/rear differential is limited slip. I am assuming that this rule goes for all differential front and back? What oil should be used in the transfer case and does limited slip effect it?

Thanks for the help Brent

[David Proske]

I use Mobil 1 syn ATF in the transmission and transfer case in my 94. I use syn 75w90 in the rear diff. I use regular 75-90 GL-5 in the front diff because it leaks a little.

When I bought the truck back in 97, I had a problem with gear oil blowing out the rear axle vent tube when cruising on the highway. The mineral gear oil was overheating. I switched to synthetic and that stopped.

I do not believe GM offered a limited slip for the 94 front axle.

Most good synthetic GL-5 oils have the additive in them for the rear axle.

[LanduytG]

If you use Amsoil Severe Gear 75W-90 you will not need and additive,even if its a limited slip.
Your diff will run 20-50 degrees cooler when running a Synthetic. Also look at the gear lube you mentioned. I am not positive but I don't think its a pure synthetic. Amsoil, Mobile, Red Line are a few that are.

Greg

[trbankii]

I'm running synthetic in both differentials, transmission, and t-case. Makes a noticible difference in shifting (manual tranny) and I can notice that things move easier when starting out in very cold weather.

[DA BIG ONE]

Originally posted by bdv:
Dear Dr. Lee

Also I have purchased Suburban used, how do I tell if front/rear differential is limited slip.
Thanks for the help Brent Front diffs in road machines are never limited slip, always open. However, lockers can be had, two types are (1)electric, then there is (2) air.

Air lockers are common in military equipment, and have been available in aftermarket for some years now, while electric lockers are somewhat newer to the aftermarket world.

I have an ARB front locker, and I can't praise it enough! No more, one wheel spinning up front when it gets air, while the one planted on the ground does nothing.

Go full synthetic, and rest assured you did all you can to protect your investment!

[tom.mcinerney]

bdv:
I think last year More Power did an article regarding rear diffs. Maybe check it out.
Pretty sure there are some threads here cautioning against syn in some rear diffs, possibly your ltd slip. Do "search" here, also try the driveline forum and 6.5L forum.

[CleviteKid]

When I put my Auburn limited slip differential in my '84 axle, the Auburn instructions said to NOT use synthetic lubes with this limited slip. In spite of that I use the Valvoline synthetic blend axle lube, and add one bottle of the GM "Limited Slip Axle Lubricant Additive" GM Part No. 1052358 (available at any Chevy or GMC dealer parts department, or via the internet from your favorite GM parts provider). This combo works great. I would like to hear if someone has run a full synthetic with the GM additive in an Auburn limited slip differential.

Dr. Lee :cool:

[DmaxMaverick]

Good Dr.

On my '85, I've had Auburn L/S diff's since the 80's. They've changed the design over the years, but it is essentially the same. The either work, or not, with no middle ground. I went through 2 carriers on the 10 bolt R/E (which I suspect you have), and finally upgraded it to a '78 12 bolt. The 12 bolt has held up for about 10 years, and has no sign of letting go.

Yes, Auburn does recommend you do not use synthetic. Like you, I tried it. I found the L/S function was marginal at best, and when heated up, like when off-roading, it failed all together. The L/S stopped transfering torque, then it just exploded, blowing its "guts" through the cover. I think this is one case where synthetic is not better. In my case, using synthetic lube was better without an additive, as most synthetic labels will suggest. The reason for the additive is negated by the formulation of the lube, and may have actually caused the synthetic (in my case) to fail.

The best combo I've found for an Auburn L/S is GM 85W-90 diff lube, with 2 bottles of GM additive. I tried other brands of dino lube, but it would fail in half the time. I change the lube at about 20K. The 12 bolt has been very happy with this, as was the first 10 bolt, before I started messing with the lube. Those rebuilds got old, and were costly. The GM/GM combo was recommended to me by a good local 4 x 4 shop that installs and services Auburn products. I should have listened to them the first time. I would be several thousand $ ahead now.

[jspringator]

My Sub has an Eaton locker, as opposed to a ltd slip. Totally different lube requirements. I think the glove box code is g80. Someone correct me if I'm wrong. I use full synthetic with no problems. Since I tow heavy, I change frequently.

[tradr1]

can anyone tell me if there is any new style of block heater avaliable for 97 sub diesel

[trbankii]

Best to start a new subject to post your block heater question rather than hijack a thread on differential oils.