I changed my rear diff fluid at 5K miles to 75w140(grape juice). I tow a 10K 5er. After reading a THOROUGH search on the subject, I can't be sure that I didn't screw up! Should I go back to the 75w90? What is the difference between the two? Thanks in advance...confused newbie.

The 140 shouldn't be a problem for you, especially if you are towing. It is synthetic, right? Even though I only tow my 6x6x12 box trailer and not very often, I put Amsoil 75w140 in my diff and ran it for about 50,000 miles with no ill effects. It was clear and clean when I dumped it to install my Mag Hytec cover at 90,000 and I refilled with 75w90 Amsoil. As long as it's a good synthetic, the difference in weight should be no problem.

I would change it back to the 75-90 because I think the 140 is too thick. Thicker fluid has a harder time getting between the gears to lubricate properly. The differential backlash ( clearence between ring and pinion) set up by the factory is tighter and therefore as not compatible with the thick fluid. Just my opine.
mike

I actually have to disagree on the 140 lubricating less than 90. I actually had the 140 on hand because an "expert" told me that I should definitely use 140 as a minimum in my previous truck ('99 Tahoe 6.5TD) with a 9.5 inch 1500 diff. Since I had it in the garage and it was a factory approved "option" for rear lube I went with it. When I pulled my cover in April and installed the Mag Hytec, there was the least amount of metal dust on the drain plug magnet that I've ever seen. The interior of the diff and the locker and ring gear looked "factory fresh" and the drained lube was still amber and clear with very little of the black friction clutch material that shows up in the G80. I have to think that the 140 did an *excellent* job of lubricating my diff! But that is just my opinion based upon what I saw in my diff. I certainly don't think draining, discarding and replacing a recent lube addition is warranted or necessary - ESPECIALLY for someone who's towing a 10k 5er!:o:

Well, I decided to error on the cautious side. The drained fluid looked a little grey, and the magnetic plug had a small amt of "sludge" on it. I went with the Mobile 1 synthetic and used the right method this time. Instead of making a mess with a small flexible funnel, I bought one of those 12" extensions($4) at WalleyWorld that screw onto qt. bottles. After positioning it above the fill hole, I found that loosening the extension from the bottle a couple of threads helps eliminate the seal and it went in FAST! No spills or mess! Myrtle Beach...here we come! Thanks again for the input/reading everyone.:)

The owners manual says to use the 75-140 synthetic when towing in extream conditions otherwise the 75-90 synthetic is standard, front and rear. correction the early model front diff is regular oil, because the breather plug rots with synthetic

I have had much better success with 75/140 in the older gm 14 bolts than with 75/90. Both oils were synthetic, much more metal with the 75/90 mobil 1 than with the 75/140 redline. I tow very heavy loads and work the rear end hard, and would never go back to the lighter oil. The heavier oil is providing a thicker cushion to keep the teeth from gauling. 75/140 synthetic is the recomended oil by all of the big three for trucks that will see heavy hauling/towing. The only down side is that there is more drag when the temp drops below 0 F.;)

The owners manual says to use the 75-140 synthetic when towing in extream conditions otherwise the 75-90 synthetic is standard, front and rear. correction the early model front diff is regular oil, because the breather plug rots with synthetic

The vent plug does not "ROT":rolleyes: - see here:

http://www.dieselplace.com/forum/showthread.php?t=34662

how would 85w-140 would work with some lucas in it ?

I've got both GM bottles here, and they pour almost exactly the same cold. The 140 just won't thin out as much. I use the 140 in both, and in all my other trucks and never had a problem. i'd stay with the 140 without hesitation.

I would stay away from the lucas stuff and use some mobil 1 or GM "grape juice" or what ever synthetic equivilent in what ever weight is appropriate. I run 75 90 mobil 1 but I dont tow.

If 2 oils have the same smaller # they should pour the same when cold right? I can never keep all those #'s and such straight......

In the previous posts I noticed that people say "in the manual" it says to use 75w-140 if towing heavy loads. I have an '03 D/A and I can't find that anywhere. Can somebody tell me what page or what area of the manual it says this? Thanks for the help.

I would change it back to the 75-90 because I think the 140 is too thick. Thicker fluid has a harder time getting between the gears to lubricate properly. The differential backlash ( clearence between ring and pinion) set up by the factory is tighter and therefore as not compatible with the thick fluid. Just my opine.
mike

It is my understanding that both 75W90 and 75W140 are both 75W oils when cold, but change to either 90W or 140W when warmed up. Due to all the conflicting opinions, I stayed with the Mobil1 75W90, even though I do tow. I am going to monitor the condition of the oil, and if it looks diry too soon, then maybe I'll change to the heavier for the rear. I just don't see where running the heavier oil would be a problem, even in colder temps and not towing. But I would definitely use a synthetic. I too couldn't find any info in the owner's manual. If it's there, I missed it.

I can't find anywhere in my owners manual either that it would call for anything other than 75w 90 synthetic, so I'd be interested if someone's found that (in an '05 owners manual). If you choose to use something other than what GM spec's, you run the risk of denied warrantee if there's a problem and they find out (a simple oil analysis will tell them).

The original poster already changed back, so that's why the "dead horse" title. However, as one poster pointed out, two multiviscosity oils with an initial number being the same have the same measured base oil weight, and would flow the same cold.

what causes the oil to get thicker on a multi weight oil is polymers, which don't contribute to lubrication, btw. Thus the greater the difference between the two numbers, the greater the polymer content. In diff fluid, one of the principal high pressure lubricants is the additive zinc dithiosulphate, which actually penetrates the metal to a several micron depth. And as far as the oil sticking to the gears, it's an anti-sling additive that does most of the work here.

Thanks, but I'll stick with what GM calls for, no matter what I tow.
But then again, what do I know...

I have set up ALOT of truck rear axles but never set-up a new 11.5 gm 80 rear end yet BUT the 1/2 tons in 99 went to a 2 step flank cut process where they tell you to set the back lash at .003-.006 instead of the .006-.010. This may also be true on the gm80(hence the "tighter backlash" mentioned earlier).Smitty