Some time back the SAE viscosity range was changed to where the 75w90’s can be “thinner” now. A current 75w110 is about as “thick” at 100*C as the “old” 75w90/80w90’s. The current cSt range for 75w90’s is from around 14 cSt to 17 or so at 100*C. Most off the shelf 75w90’s are in the 14-15 range. I think the upper end of the 75w90 & 80w90’s used to go up to around 20 cSt at 100*C, but that void has been filled with 75w110. The higher the number the “thicker” it is. I assume 75w90 is still ok to use for a street car. Or is it better to go up to a 75w110 or the more readily available 75w140?

Another turn down the rabbit hole takes me to synthetic vs “conventional,” and finding an oil that doesn’t contain limited slip additives. I have the Torsen rear end and Torsen says that using non synthetic and no limited slip friction modifiers achieves the best torque bias ratio. GM says to use synthetic but I’m assuming a big part of that is due to drain intervals. Torsen also says that a “thicker” gear oil can slightly reduce the torque bias ratio, but to use oil that’s “thick enough” to protect the ring & pinion.

Almost every gear oil on the market contains friction modifiers (limited slip additives) now days. Even most of the conventional 80w90’s have them already in it now. Even Walmart Supertech 80w90 has it. Even if it doesn’t day it in bold print on the front of the bottle if you dig deeper you’ll find out that it does. AC Delco’s 75w90 has it, Valvoline’s 75w90 & 80w90 all have it. Pretty much every brand has it now. Even Lucas 75w90 has it. There are only a few that don’t like Mobil Delvac 1 and Redline NS. But they’re expensive & not all that handy to come by at the store.

I changed the rear gear oil in mine today and I used a conventional 80w90 that doesn’t contain any limited slip additives. It’s a little thinner than at 100*C than GM’s Delco synthetic 75w90 but I think it’s close enough to be comparable. One thing that the synthetic will probably edge out the conventional is the temp range. How hot do differentials get?

I ran it down the road a little including some WOT down shifts from 4th to 3rd (auto) at Hwy speeds and the gears were quiet. Both accelerating and and coasting. I didn’t notice any weird noises from the differential when making slow speed turns either. I may run this oil until the summer time and then I might go with Mobil delvac 75w90. It’s a synthetic and it’s one of the “heavier” 75w90’s. It’s also one of the few that doesn’t contain limited slip additives. I had to dig deep to find even an 80w90 that didn’t have any. I’m using John Deere 80w90 GL-5. It’s made by Chevron and it’s API GL-5, API MT-1, and SAE J2360 rated. They also have an 85w140 which might can handle heat better, but it might be getting on the “thicker” side of things.

I’m going to attach a screen shot of Lucas synthetic 75w90’s specs where it mentions limited slip. It doesn’t say that it has it
in there, but when it says “suitable for limited LSD” that means it’s in there. If you reach out to the MFG they’ll probably tell.

 

I don't think it matters if you use a synthetic or conventional with the stock GM Torsen rear. The T2 and other "upgrade/aftermarket" Torsens might be different, as mentioned by Torsen directly, but the one that came stock in your car would have been filled on the GM assembly line with 75w90 (of the day) synthetic, which was also GM's service recommendation. The GM owner's manual called for the LSD additive (friction modifier) even in the '99+ cars with a Torsen, but I've never understood why as there is no friction to modify in this style of LSD. You can leave it out with no ill side-effects, many folks have. I can't speak for the current GM/Delco/Goodwrench 75w90 synthetic lube, but I can tell you that 20+ years ago it did NOT contain the additive as a premix.

On the other hand, for those of us with stock Auburn LSDs (1998 and older V8s) it's very important to stick to 80w90 conventional, and to use the LSD additive for best operation (meaning seamless operation in the corners). If you omit the additive, it will usually get clunky and shudder when turning, etc. I wasn't aware that the 80w90 is no longer as "thick" as it used to be, so maybe it would be better to use w110 *if you can get it in a conventional*. Lots of folks have reported poor results using synthetic lube with the stock Auburn rear, so I'd rather not change the grade if that means having to use a synthetic - but for you, this doesn't matter as you don't have an Auburn.

 

When I bought my S60 with the S trac from Strange they sent the 85w140 Lucas. They recommend the thicker non synthetic fluid.

 

Also, after a new ring & pinion install how much is too much metal on the diff cover magnet during the first fluid change?
This is after about 1k miles, which I realize is probably a little long.

 

^ Doesn't look too terrible to me. FWIW, the first diff fluid change isn't recommended until 7,500 miles in the owner's manual.

 

Good deal. Years ago I had a rear end rebuilt in a Ford and I believe they said to change the fluid after 500 miles or so.

 

I changed the fluid in my 10 bolt at 22 years and 105,000 miles. My Silverado is on original diff fluid at 145,000 miles.

 

What's interesting is that GM only recommended that initial 7,500 mile (6,000 mile for city use) diff fluid change for the '98s, which still had the Auburn rear. Looking at the manual for my '02 Z28 with stock Torsen, it doesn't actually recommend any diff fluid changes at all - at least not up to 150k when the maintenance schedule stops. I guess they figured the clutch style Auburn was more sensitive to any assembly or break-in debris that might have been left behind. But even the Auburn cars don't get any further recommended changes after that initial change.

 

This is a good thread for me, as I just bought a Detroit Truetrac for my S60 and it says to use NO limited slip additive. Looks like I'll be searching out some conventional WITHOUT limited slip additive.

 

I’m pretty sure that Mobil Delvac doesn’t have limited slip additives. There is a Mobil LS that has it though.
Delvac

 

Eaton says to not use synthetic because it's too slippery.

 

I wonder how much of that is true (or still true) and how much of that is an old hang on from the days of yore? People were leery of synthetics back in the “old days.” The vast majority of synthetic oils are made with non-synthetic base stocks (oil).

 

It's still true with the Auburn. I've corresponded with folks on here in recent years who have made the mistake of putting synthetic in their factory/GM Auburn and immediately noticed chattering/slipping/noises that weren't present with conventional. Keep in mind, most of the stock rears have a ton of miles on them now so they are more worn and probably even more sensitive to this than when they were new. Even still, this topic was discussed many times back when these cars were new as well, and some folks had trouble with synthetics + Auburn even back then.

One thing of note that I do remember from the old days. The factory Auburn was usually OK with the GM/Goodwrench synthetic of the day - probably because this was closer to a conventional lube than some of the more high dollar synthetics.

 

My Yukon Dura-Grip differentials all shipped with instructions clearly recommending NOT to use synthetic gear lube.
I have 3 Dura-Grips in service, all 3 are in 1/2 ton GMT800 platform 8.6" light duty axles.
Dura-Grip insalls in (year) 2014, 2016, and 2019.

All 3 units were initially filled with DELO synthetic 75W-90, and same fluid at each change interval. All 3 have been beat like like a teenager's first car, and no failures as of yet.
The 2016 Dura-Grip install has been through 3 canadian provinces, and 11 states from Texas to Alaska, with 4.56 gears and Magnuson blowing 11 PSI- I bounce off the rev limiter every time I drive it.
The 2019 is in a Suburban with Turbocharger and I make damn sure it hits 190kPa through 3 gears every time I drive it.

" When I do break something- it will NOT be blamed on synthetic lubricants."

 

Straight from the application guide on their site:

"Synthetic oils are not recommended for Truetrac
applications. While some customers have found that
synthetic oils works well in this application, Eaton has
not confirmed under test conditions that all synthetic oils are
compatible with Eaton Truetrac applications."

https://www.eaton.com/content/dam/ea...n-guide-en.pdf

 

Whats wild is the friction modifier or “limited slip additive” that one is supposed to add to limited slip diffs that use clutches to make the differential supply power to both wheels is added so the clutches won’t chatter. If one was to use conventional without adding the friction modifier the clutch packs will chatter because they’re not slipping enough. Think about it like wet sandpaper vs dry sandpaper. The bottle of LSD additive is the “water on the sandpaper” and the clutch packs are the sandpaper in this scenario. The more friction modifier the less the clutches will “grab.” If one used conventional or synthetic w/out friction modifiers (LSD additive) the differential would “lock” better but the clutch packs would wear out quickly and they’d be noisy. The additive is added to allow the clutch packs to slip some. I bet if one added a butt-load of LSD additive to an LSD it would turn the LSD into an open differential. But I bet it’d be quiet.
I’ve heard of guys that have a Ford Trac-Lok, which is an LSD with clutch packs and an S spring, add very little additive at a time. They’ll drive around the block to see if it’s chattering and if so they’ll add a little more and to try again. They don’t want so much friction modifier added in to where it affects the “locking action.” I’ve always just dumped the entire 4 oz bottle in and called it good.

If the clutches in a differential are chattering I don’t think it’s because the fluid was too “slick.” I think that sound means that it’s not “slick” enough. Maybe the synthetic GL-5 they used didn’t have any or not enough friction modifiers in it, or their differential was worn out.

 

Auburn works as a locked rear when the wheels are unloaded (if you lift the rear of the car and spin one wheel, the other will spin in the same direction; a Torsen has the opposite default state, meaning it acts as an open rear [wheels spinning opposite] when unloaded). Friction modifier does exactly as described; allows for seamless unlocking (slip) during curves and turns, vs. when driving straight with locked holding power. Some folks have discovered that you can get a little more life out of a dying Auburn by omitting the friction modifier so it stays locked "harder". Of course this is a double-edged sword as it's also harder on the rear when it needs to unlock/slip, such as during a turn. But if it's on the way out anyway, it's a trick that can work.

The clutch surface is designed for a specific amount of friction, thus a very specific lube and amount of LSD additive is specified. If you use a better (more "slippery") lube, it would be like adding extra friction modifier - meaning less holding power when you WANT it to hold (thus more chatter, unless it slips SO much that it stops chattering and just acts as an open rear). I think the only time you'd hear chatter withOUT the friction modifier would be during curves/turns when it can't release/slip as smoothly as designed (vs. when filled with the LSD additive).

I think the Delco/GM Goodwrench synthetic lubes have worked out OK for lots of folks using clutch-style LSDs over the years because they are closer to the specs of conventional lube than the top name synthetics.

 

I believe the chattering is because it’s not slippery enough, thus they can’t let go as easily. “The two pieces of heavy grit sandpaper that you’re rubbing together are dry so they can’t slide against each other they easy.” They stick and slip off. If they were slick they’d slide against each other and would be a little less noisy. Just like you said, the friction modifier makes the conventional oil a little slicker. Mo slick (lsss friction) equals less noise. I believe GM may have recommend adding a friction modifier with the Torsen at some point to reduce the “rusty screw noise” that can happen when the Torsen “locks up.” I think it may have been a TSB. I’ve never noticed that noise though. But it’s a trade off since the slickness can reduce the torque bias ratio.

I used to think that the friction modifier that you’d add was to “improve grip.” But it’s sort of the opposite. But it allows smoother engagement, longer clutch life, and less noise. On a clutch pack & spring style LSD anyway.

 

Well, you don't want it to be too "slippery" OR too "grippy" because either way will lead to unwanted performance issues. Random locking/unlocking for any reason could potentially lead to chatter (noise), though it's only going to be a shudder that you can actually *feel* if it's gripping when it shouldn't be (into a turn, etc.). All else being equal, synthetic is a better lubricant and seems to act as an additional friction modifier when used in the Auburn. Maybe if you omitted the additive and used just the synthetic it might balance out and work OK, but why re-invent the wheel? If you use the factory recommendation of conventional + 4oz of LSD additive it will work perfectly (assuming the unit isn't worn beyond it's service life).

With that said, I realize that yours is a Torsen so the "conventional vs. synthetic" debate doesn't really matter in your application.

But you are correct in your current understanding of the additive. It's not to "improve" grip strength. Here is the actual quote from the back of the GM LSD additive bottle (Part #1052358):

"GM LIMITED SLIP LUBRICANT ADDITIVE is a specially compounded formulation which when used in limited slip equipped rear axles tends to * Modify excess clutch plate friction * Facilitate applying and releasing of clutch plates."

The bolded part is paramount. This product "modifies excess friction" (i.e. reduces) when appropriate.