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What kind of differential damage have you seen due to wrong gear oil . . .?

Hey, guys

I recently worked on a truck that had a very severe problem with the rear differential, rendering the truck undrivable

I removed the cover and drained the fluid into a clean tub for inspection and analysis

A horrendous amount of metal came out along with the fluid, and the pinion gear was wasted.

A quick records check revealed that somebody had done a diff service, using 85w140 conventional instead of the 75w90 synthetic which it was supposed to get. This diff service was just a few days ago, by the way.

I looked on the dana industrial website, specifically the failure analysis section. And the “incorrect lube” pictures pretty much match what I’m dealing with.

Have you guys ever seen incorrect gear oil destroy a differential in only a matter of days?

In case you’re wondering, this vehicle has just just under 90K and there was absolutely no history of diff troubles before this.

And it’s a class 1 truck, not something big and heavy

The vehicle will get repaired, so junking it’s not really a subject for discussion.

I’m basically looking for opinions and stories that relate to similar experiences and/or incidents

Dexcool used in anything.


I am not sure why Dex Cool got mentioned as that is the coolant. Maybe it was meant for another thread. I have done that before when many tabs are open.

That is very interesting about the wrong lube destroying the diff. I sure wouldn’t have expected that to happen. Maybe it wasn’t thin enough to properly lubricate or something. I guess it could be coincidence as well. It would have been interesting to see the old fluid in case it was already starting to eat itself.

Was the fluid used even gear oil? Maybe something completely different was used. I have seen the wrong gear oil used and gear oil that was so full of water that the insides of the diff were all rusty but they still worked!

It seems the real gear oil always has that stink associated with the high pressure anti-wear additives. Did this stink?


The OP edited the original title from, “What kind of damage have you seen due to wrong fluid?”




As I said, the guy incorrectly used 85w140 conventional gear oil, instead of 75w90 synthetic gear oil that the vehicle was supposed to get

I even watched him do it and told him to his face “You’re using the wrong fluid”

But I’m just his co-worker, not his boss, so I have no authority over him

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I see. You gotta love people! If they can mess something up, they will…

So you were there? Did the old fluid look like anything was wrong? I know that some manufacturers are now real sticklers about using synthetic fluids as well. I just can’t see it making that big of a difference over such a short time but maybe it does. Maybe it was thicker/couldn’t flow to where it needed to be.

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It seems very unlikely that the non synthetic fluid would have caused any immediate damage. In fact it would be likely that the oil you were charged for would have been an adequate grade for any normal use in temperatures above 0* F. Straight 90 weight gear oil is comparable to 40W engine oil in viscosity and the critical difference in rear end lube and engine oil is the extreme pressure additives required to deal with hypoid gear stress.

I have never seen any damage done to a rear end due to a wrong viscosity oil but very few mechanics that I know use anything but the same old 90W GL5 that has been as common as Morton salt for quite a while. Of course manual transmissions are a different story since some newer models REQUIRE much thinner vehicle specific oils, some even require ATFs while older models were somewhat universal in the use of 90W GL3/GL4.

When the pinion gear disintegrates in a differential it is usually the result of worn pinion bearings or carrier bearing or extreme overloading such as driving in LOW RANGE in a 4 wheel drive shifted to 2 wheel drive. .Of course that requires some modification or some heavy duty jamming.

I’ll say

That 85w140 conventional is like molasses, compared to the 75w90 synthetic


Did it seem to be 85w140 gear oil that came out? If so, either the differential was near death before the prior service – maybe that’s why they tried a drain and fill service on it – or the thicker gear oil viscosity at higher temperatures just couldn’t do the required lube job. The latter hypothesis seems hard to swallow that such severe damage could happen in a few days, so my theory is the differential was already in its death throes prior to having the wrong spec gear oil added.

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Maybe that was it if it was that much thicker. Maybe there is a reason synthetic is specified.

You mention that the guy used the wrong fluid… Did he use the specified amount of the wrong fluid or the wrong amount of that as well? Maybe it was low but I know people who have basically run their diffs dry and they don’t come apart like this.

I think (and admit I could be wrong) that the lighter weight synthetic gear oil is recommended more for better fuel economy than anything else. I’ve always used what was recommended for fear that my hypothesis could be incorrect. Either way, you’d think the thicker than spec oil could possibly cause more heat and maybe not reach some places that the thinner oil could. Which could result in damage, but it seems like the damage would occur after many miles. I don’t see how thicker oil could directly affect the ring and pinion at all. Bearings, I could see possibly being damaged from heat or lack of lube over time.

I’m starting to wonder if they didn’t use thicker oil to try and mask some noise in the differential. And the damage was already being done before the thicker oil was put in.



it was the factory 75w90

I know that, because I checked our records, and it had never been serviced

I doubt it . . . because I’m very familiar with the vehicle in question. And there are plenty of other trucks in our fleet, exact same model, same model year, same drive train, same vehicle operators, similar mileage, that have had absolutely no differential problems. The only difference was the guy servicing the diff and using the wrong fluid

No, it was never low, not before and not afterwards

When it first came in, he removed the fill plug and the level was correct

He refilled it to the correct level . . . with the wrong fluid

And when I worked on it a few days later . . . it came in specifically because it was undrivable . . . the fluid level was correct

No . . . the guy used the wrong fluid because he assumed 85w140 conventional would be fine. He didn’t bother to look it up, and when I saw him putting in the wrong fluid I told him to his face that he was using the wrong fluid

The truck didn’t come in with any odd noises or improper differential operation . . . not when he serviced it a few days ago. It simply came in for a regularly scheduled service

But when I got it, it specifically came in for differential problems.

I know all this because it’s one of our fleet vehicles. There’s no way that somebody had the diff serviced elsewhere in secret

Welp, I’m not a true mechanic. But I’m going to go ahead and say running 85w140 instead of 75w90 for a few days- I just don’t think that damaged a pinion gear. If it did, I hope someone explains how.

Got any pics? How do the carrier bearings and spider gears look? The faces of the ring gear? I would expect wear/damage on all those if the fluid was at fault.

Is it possible that there was some noise from the rear diff that the driver did not report before the maintenance? Like the pinion bearings were marred already before the service? Is a test drive part of the procedure before any PM at your shop? Could Tech A not have noticed the noise, or did notice it and not care?

I would have a hard time believing that the incorrect viscosity would cause that degree of damage in virtually no time at all, especially in your climate. It’s not like it was 10 degrees one morning and the lube turned into Jello. You’re in a pretty mild climate. Now if the truck lived in the Canadian Rockies and went a couple of winters with the 85W140, I could see it.

I thought you said in the OP it had just recently been serviced using 80w140.

They’re also bad, but not nearly as bad as the pinion

Yup, it’s clearly marked as such on inspection sheet

Certainly possible. Hard to say. I specifically turn the radio OFF when I’m going on a road test, because I want to get a real feel for the vehicle in question. But I know for a fact some of my coworkers really crank up the radio.

It had never been serviced before last week

So it had the correct fluid until last week

Then the guy put in 85w140 conventional, as opposed to the 75w90 synthetic that was called for

Well, fortunately for all, it’s a fleet vehicle and it will definitely get repaired.

I did make sure to let the guy know that the truck he serviced with the wrong diff fluid has now come back with a wrecked diff.

Even if nobody is to blame for the wrecked diff, everybody deserves to know what’s going on

It was clearly 85w140, the same 85w140 the guy incorrectly put in last week

Our 85w140 is thick and green, our 75w90 is more yellow and quite thin, in comparison

Even if you were color blind, you could tell by feel that it was too thick for the application. That is, if you looked up the specs and knew it was supposed to have 75w90. and I always look up the specs

So my question was, when you drained the gear oil out, did it seem like it was 85w140 vs 75w90 syn, or couldn’t you tell just by looking at it and by feel?

No matter what I guess there will be strict orders in the shop to ONLY use the SPECIFIED fluids! I still find it strange when you have known people who know when they need to add fluid to their own differentials when they start to whine and seem to drive them without issues for quite a long time this way.

Modern tolerances are probably a lot closer and that thicker fluid just didn’t cut it.

One thing is for sure. Some people guarantee job security you have to go around and fix all the problems caused by others. I see it in my line of work and everyone in any service business sees the same exact type of thing. Sometime it is the home repairer but sometimes it is a so-called professional as in this situation. I had one this week where over half my day was spent cleaning up the mess caused by someone else so that I could then fix it correctly. I mean things were just blatantly done WRONG and likely to cause more problems down the road. I see lots of problems caused by the guy they had used before me so this didn’t surprise me. The only reason he stays in business is because he charges 1/2 of my price or anyone else for that matter. You get what you pay for!