Down the Gear Oil rabbit hole again: for Eaton Torsen “upgrade”

[Impatient]

I’ve read numerous threads on BITOG, F150, FordTransitUSAforum, etc, as well as Eaton’s own website, and am now more confused than before. . Does anyone have a 2023 opinion of which gear oil might be best for the Eaton TrueTrac? Any real life experience from someone who has tried both conventional & synthetic? Or an alternative Torsen for the Ford 9.75? I know this has been beaten to death, but consensus still might be lacking.

Vehicle: Ford Transit 250 2wd EcoBoost van, total weight with camper van buildout and contents: 7500lbs. Original factory 3.31 open diff…standard Ford 9.75 internals, original 75w85 Motorcraft synthetic gear oil. 78k miles. Diff began leaking at the cover (RTV “gasket”). Used for “off-road” camping! Did I say “open differential?” . It’s probably time to change out the fluid anyway.

bare minimum is to get the leak fixed (~$350, since I hurt my back, and shouldn’t attempt DIY). Opportunity is to upgrade the “1-wheel-drive“ open diff to Torsen LSD (~$1200-1400 all-in), so some incentive to upgrade now, and eliminate duplicate work. I don’t see any other upgrade (elocker, clutch-based, etc) making sense, as 75+% miles are highway, maybe 10% dirt forest service roads or other traction-challenging surfaces, and ice. It’s just a matter of time though before mud or sand, snow or rocks trips me up.…and I travel solo. (There were many times I wanted to “go further” off the beaten path but prudence dictated I hold back.)

Apparently Eaton still clings to recommending 80w-90 conventional gear oil, while Ford recommends synthetic 75w-85…unless you want to use 75w-140, and synthetic unless you want to have 3000 mi OCI’s (per 2015 OM, the 2018 OM deletes that 75w-140 & conventional confusion: Motorcraft synthetic 75w-85 only, and not before 150k miles ). 3rd party interpretations are that friction modifiers are bad for Torsen performance (reduced bias), and synthetics are guilty by association since most nowadays try to incorporate FM’s whether you need them or not. but supposedly internal friction is your friend in a Torsen.

2 local shops insist 75w-140, but are OK with synthetic. One is a “differential“ specialty shop (With a bit of an attitude). So between the local good-ole-boy “thickie” recommendations (hey, they probably do know more than I do), and the Eaton “old-school” conventional 80w-90 recommendation, it’s hard to push for a 75w-90 Amsoil (or Motul, or HPL) synthetic option.

I originally thought Motul Gear 75w-90 would be great: closest to stock viscosity, as it’s thinner than most 75w-90’s, and no FM’s. Then I read @SubieRubyRoo ’s 5 gear oil comparison, and thought Amsoil’s 75w-90 sounded even better (it really does!!!). Of course HPL claims their Diff Life is best (it also looks very good in the comparison, though not a hands-down no-contest best option). Then I read @MolaKule ’s repeated plug for Amsoil 75w-110, and the local and F150 forum penchant for 75w-140, and, and, .

I saw where this very synthetic vs conventional debate regarding TruTracs was occurring here on BITOG in 2008 with @Jim Allen. 2008!!! Then saw that the Eaton Torsen dates back waay before that, and it made me wonder how much, if any current R&D or testing is Eaton doing? They claim they can’t recommend synthetics because they haven’t thoroughly tested all of them. Well, what have they been doing for 30 years??? They haven’t even published a spec that oil co’s could strive to achieve, or get a license for, from what I can tell (though I do see Eaton specs claimed by oils, just no mention if that spec is for the TruTrac). Of course, Google won’t certify that my search is all-inclusive (the rabbit hole is too deep).

While M1 Delvac might be OK, Motul Gear 300 a lightweight over-achiever, some Amsoil Severe Gear possibly great option (though they now have FM’s), possibly Redline, and HPL most expensive (and has FM’s). I assume Valvoline and non-Delvac M1 are the kind of synthetics Eaton doesn’t want to recommend (Because FM’s). And if there is a top super-quality conventional, I’m all ears. Your thoughts?

 

[SubieRubyRoo]

I’ll keep it simple, since in this case (stock vehicle with Torsen), it really is simple:

Pick whichever one you like… the testing that was done for my gear oil thread you reference shows that, aside from things like oxidative stability, there’s not much other than personal preference to distinguish between them, on the surface. So, whatever brand warms your heart the most, choose that one since here you’re not asking it to deal with huge shock loads or even demanding conditions.

As far as oil grade, 75w90 is plenty capable especially if temps are kept in check. Are you using an aftermarket diff cover? That should help. I guess the only real downside to stepping up to 75w140 will be more HP loss due to thicker fluid (and possibly a slight MPG hit) but that’s about it. In a sub-400HP, auto trans vehicle I really think you’d be fine with the 75w90 or maybe step up to the 75w110.

 

[Impatient]

So, you don’t see any issue with synthetics being “too slippery” for the Torsen? I think that was one criticism. Sounds bogus to me, but seriously, that was one of the main objections that has stood out in the 2008 debates, and others. I was hoping “newer” verified information was available. Unfortunately, I think it’s become more common for synthetic gear oils to include the FM’s “in case you need it,” Even Amsoil, and I think even HPL. And this is a case where maybe you really don’t want it.

part of the challenge will be getting a “more knowledgeable” shop to not force their choice on me. Two have already said, if I want them to do it, they will only do it their way. somewhat understandable… But the more “real” differential shop didn’t believe me that Ford was putting 75w85 from factory, so credibility is already a concern. The good news is they use Amsoil (but only 75w140).

I have thought of asking them to add a drain plug to the cover, but I bet they would try to sell me a fancy cover instead…unfortunately there is no room as the (fairly worn 225/75r16) spare tire is already within 3/4” as it is…and I really should have a larger spare if I go up in tire size (but that’s a whole ‘nother discussion).

 

[wrcsixeight]

Overanalysis paralysis.

Been there, done that, will do it again.

You've likely aleady annnoyed the shops enough, that they would shorten the amount of time and care they'd put into any work you'd have them do.

Seems the only issue is the diff cover gasket is leaKing.
What, 10 bolts, a 7$ paper gasket and or gear oil rated rtv and an hour or 3 and the fluid of your choice.
Find a compentent driveline shop that you have not yet totally annoyed.

I'd love not having an open diff.
I've been there, wishing for more traction, wanting to get farther away from the 2wdrivers.

I'd air down, use carry chains, recovery gear, walk the questionable route first, use momentum when possible, anticipate every dip where Id only have one wheel compromised drive, and be thoroughly surprised at where I could make it to, and still ....
It was not far away enough.

The Torsen is completely optional.

 

[dnewton3]

It seems to me that the OP actually has a good grasp on all the information. Problem is ... some is good and some is not so good.

In this case, defer to the OE maker of the diff; EATON (which owns the Detroit brand line).


It is true; torsen type diffs are sensitive to the friction modifiers and base stocks used in some lubes because it affects the torque bias. Some syn fluids and fluids with FMs can negatively affect the friction loading of the helical gears in the diff.

You say you've spent a lot of time looking this over from all angles. I would say, if that's true, then go with the OE recommendations.
I have a Torsen diff in my MX-5. I have used proper, non-syn/non-FM fluids and found that the bias is exceptionally good. I've not tried syns in the diff; that's because they are not optimum for helical-gear diffs. I don't want a fluid that "will work ok". I want the proper fluid that gives me the highest bias; that being a conventional GL-5 with no friction modifiers added.

In this case, follow what Eaton says and not Ford.
https://www.eaton.com/content/dam/e...control/Eaton-differentials-owners-manual.pdf
Pay close attention to page 8 where they specifically discuss the lube recommendations for the Truetrac; here's a snapshot:

You can use a syn, and/or something with FMs, but what will happen is you'll lose most of the torque bias you're paying for. Why spend all the money to upgrade to a helical-gear diff, and then lose the very benefits you're paying for??? Whereas you can get 3:1 or even 3.5:1 bias using the right fluids, you may drop to 2:1 or even less using the wrong fluids.

The best bias comes from using a GL5 mineral base lube, with no FMs.

To be totally clear and precise, it's not that syns or FMs will hurt the differential; they won't cause physical harm to gears, carrier or such. But what syn fluids and/of FMs will do is harm the performance of the diff. Syns and FMs can make the helical gears too slippery and in turn that reduces the torque bias you seek.

Luckly, in this case, you can use an inexpensive GL-5; even the basic ST would work well here.
Or, Valvoline conventional HP gear oil does not contain any FMs as well.
https://sharena21.springcm.com/Publ...3793b338/510adab1-8a54-eb11-b800-48df3793b338

 

[dfarr67]

I run two of the trutracs- with synthetic, if there is a problem I wouldn't know- function and quiet operation haven't changed.

 

[dnewton3]

I run two of the trutracs- with synthetic, if there is a problem I wouldn't know- function and quiet operation haven't changed.

The problem is that you likely are experiencing some amount of bias loss. How much? Hard to tell because without an intricate chassis test rig, you've got no way to "test" the loss; something you and I probably don't have access to. Helical type diffs will certainly function with syns and FM in them; just not to their full potential.

 

[DuckRyder]

Welcome to the Rabbit hole, i think i left a scrap of my notes down there could you bring them up when you are done...

I have an Eaton Detroit Truetrac in the F100, its had both conventional and Mobil Delvac 1 in it. Absolutely no difference in performance, it currently has Delvac 1 in it, i'll continue to run that.

You also might find this interesting He works for Eaton and talks about the lubricant toward the end. (around 4:54 to the end)

 

[dfarr67]

The problem is that you likely are experiencing some amount of bias loss. How much? Hard to tell because without an intricate chassis test rig, you've got no way to "test" the loss; something you and I probably don't have access to. Helical type diffs will certainly function with syns and FM in them; just not to their full potential.

Depending on your location will influence the choice- I'm in central to northern Canada with severe cold weather operation. Synthetic for me.

 

[thastinger]

I have a 98 F150 4x4 with the Eaton TrueTrac in the Sterling 9.75 rear, lives behind 620HP/740Tq from a 2V 5.4 with a Whipple 2.9L blower making 18Lbs of boost. I went through the BS from them myself on the fluid. Have been running SuperTech 75-140 in it for the last 10 years, without issue, it locks up just fine, I drag race it on clay/sand tracks. No issues at all and it works great.

 

[dnewton3]

I have an Eaton Detroit Truetrac in the F100, its had both conventional and Mobil Delvac 1 in it. Absolutely no difference in performance, it currently has Delvac 1 in it, i'll continue to run that.

I appreciate that you believe that, but how can you prove that? Have you run statistical, controlled lab or chassis dyno tests to measure the torque bias with both conventional and synthetic lubes? If so, would you be kind enough to post all the raw data so we can see it?

I seriously doubt there is "no difference" in reality. The question is this: is there a measurable difference, and how can that be quantified?


BTW ... the Eaton rep stating that both mineral and synthetic fluids work very well is NOT the same thing as say both fluids perform identically. Don't imply your point into his statement. He merely stated that both types of fluids work well. He didn't say they both worked equally.

 

[DuckRyder]

I seriously doubt there is "no difference" in reality.

Well whether you believe it or not is inmaterial to me. You know good and well i haven't done instrumented test (unless you call it leaves 2 black marks just as long with Delvac as without it, a test), nor have you, so until you have, your doubt is just as doubtful as my actual experience.

There is no difference.

 

[dnewton3]

You claim there is "no difference". And that's quite possibly true, in your application. But that does not make it true for all applications, or the OPs application. It is merely a statement of your perception; there's no data to back it up. Doesn't make you wrong, but certainly doesn't make you right.

Generally, it is accepted that he who makes the claim must prove the claim.
It's not up to me to disprove it.

What is true is that Eaton clearly states that synthetic lubes are "not recommended" for the Truetrac applications. They acknowledge that syns will work, but they are not the optimum.

 

[Impatient]

Well, I’m glad there’s still debate, so the horse I’m beating is not quite dead yet.

Given that the most qualified local shop i am aware of told me they will only use Amsoil when installing a TruTrac, i guess I have an incentive to go synthetic. I did try to push back a little on his insistence on 75w-140 though, but only briefly.

Backing up a bit, my preconception is if I were to go the “conventional” route, I would be draining and refilling annually…or if I followed Ford‘s owners manual, 4x annually (in their differential, with their innards). Can we address that preconception? In other words, what sort of OCI should I hope for? There must still be shops that will do conventional with the TruTrac, maybe there is one somewhere near me, but this place promoting Amsoil keeps getting pointed toward in my inquiries. and I’m not going to “annoy” some shop in California or Pennsylvania because I’m not going to drive 2000+ miles round trip just to get this installed.

and wrcsixeight, if you click on my “avatar” it will present you with an option to ignore me. Go ahead, be my guest. To others who may find me annoying, sorry, but I was trying to show why it’s still unresolved in my mind…and really, see if there was any “new“ information.

 

[bdcardinal]

My Mustang came with a Torsen T-2 fron the factory and Ford recommends 75W140. Now the Motorcraft stuff does not come with modifier, Torsen and the e-locker diffs don't get a modifier.

 

[dnewton3]

Backing up a bit, my preconception is if I were to go the “conventional” route, I would be draining and refilling annually…or if I followed Ford‘s owners manual, 4x annually (in their differential, with their innards). Can we address that preconception? In other words, what sort of OCI should I hope for? There must still be shops that will do conventional with the TruTrac, maybe there is one somewhere near me, but this place promoting Amsoil keeps getting pointed toward in my inquiries.

What is your objection based on? I suspect it's residual bias (no pun intended) that somehow conventional lubes are not as "capable" as syn lubes.


You may benefit from looking at this list. This is an SAE-derived, performance-based testing system; it's not a content-based spec.
There are a slew of conventional lubes that pass the exact same demanding test; one that is based in mil-spec parameters.
Several inexpensive conventional lubes on this list you could easily get at any local store.
Further, these same lubes are approved for use in many OTR applications for up to 500,000 miles of use.


click this link, and then use the "LRI QPL" link on the left side of the page about 1/2 way down.
Also, spend some time reading the informational pages about how/why the testing is done.
https://p-r-i.org/pri-qpl/lubricant-review-institute/

 

[Impatient]

@dnewton, I didn‘t review the entire site you linked, but I have seen the LRI QPL list. So if your premise is that any (conventional) gear oil on the list is good enough, then wouldn’t that mean any SYNTHETIC gear oil on the list is good enough?

yes, I have a bias toward synthetic….mostly because I’ve been “conditioned” to believe synthetics have a better combination of consistency, protection, purity/cleanliness, longevity, hence low-deposits, and long-term wear protection. Now maybe this applies only to engine oils, not sure.

again, now I’m flailing a bit, and pardon me getting repetitive, but I’m trying to navigate between Eaton’s claims (which might not be keeping up with the times, or address only the run-of-the-mill big-name so-called synthetics which inevitably throw in some FM’s), vs the claims of some boutique synthetics. I admit, I put some faith into @SubieRubyRoo’s comparison of HPL, Amsoil, Redline, M1, and Motul. For an open Ford diff, I admit I liked Amsoil in that comparison, but for a Torsen, perhaps Redline’s other product (not in the comparison) without FM’s and the Motul bubble to the top. Still, the leading local shop insists on Amsoil. I’ve been out of town for awhile, so will resume the local search hopefully next week.

What is your objection based on? I suspect it's residual bias (no pun intended) that somehow conventional lubes are not as "capable" as syn lubes.


You may benefit from looking at this list. This is an SAE-derived, performance-based testing system; it's not a content-based spec.
slew of conventional lubes that pass the exact same demanding test; one that is based in mil-spec parameters.
Several inexpensive conventional lubes on this list you could easily get at any local store.
Further, these same lubes are approved for use in many OTR applications for up to 500,000 miles of use.

 

[pzev]

You are overthinking it, differentials are extremely robust and tolerant of a wide range of viscosites and base oils.
Also, not all synthetic GL-5 gears oils have friction modifiers, so the argument raging above regarding a reduction in "biasing" is pointless. If it were me I would gladly go with a quality synthetic 75w90 or 75w140 that was a available locally, for convenience, cost, and ease of future maintenance.

My Mustang has a torsen differential, housed in a "super" 8.8 axle, and it specs synthetic 75w85 AND an additional friction modifier. This same axle and differential has also specified 75w140 (unmodified) in prior iterations, with no trends that I could locate indicating axle failure as a result of either lubricant choice.

Go with Mola's recomendation and get some 75w110 and call it a day. Good middle ground, and should put your mind at ease.

 

[DuckRyder]

It probably bears pointing out that Torsens and Truetracs are not the same thing.

Pretty sure T2s have a clutch in them.

 

[RooflessVW]

I wouldn't drop $1000+ on some gears and ignore the recommendation of the people that made them.