75w90 or 75w140?

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Sep 19, 2022
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Yeah, I know there are others who have commented on this, but let's put forward the scenario and see what people think.

2002 1500 Suburban, 5.3L, standard G80 roller locker. Used mostly/almost always for hauling or towing; boats historically, last four-five years including a cargo/RV trailer up to 7k gross (w/weight distribution.)

It appears the diff has eaten the bearings. Light dusting of silver on the magnet, oil looked ok when drained, no "pieces" of anything in the oil, ring gear looks fine, of what I can see without removing the axles and chunk the pinion looks ok (can't tell for sure as the the chunk isn't out yet) but its got a nice growl starting at ~20mph (zero noise below that) which is not load-dependent; it makes noise at higher speeds in gear or out, accelerating or decelerating, and while there's a BIT of a difference under hard acceleration its not a lot. With the rear end on stands it you can clearly hear it once put in "D" and the "speed" gets to 20mph or above -- its definitely in the differential and not one of the wheel/axle bearings. Replacing the gear oil did nothing, so here we are and I'm going to be tearing it down, replacing all the bearings plus whatever else I don't like when it comes apart. It made the back half of a 1,000 mile trip with the noise and did not get progressively worse.

Truck has roughly 100k miles on it; gear oil has been changed on roughly 30k intervals, and always full-synthetic and "as recommended", meaning synthetic 75w90.

It LOOKS like I caught this before the damage wound up destroying the gearset but I don't feel like doing this job again. Once I have the chunk and pinion out I'll see if anything looks trashed there, but at first blush I suspect its bearings-only in terms of what's dead and needs to be replaced.

So...... do I run the 75w90 or do I go to 75w140, assuming both options are full-synthetic? The latter should flow the same at lower temperatures but since its full-synthetic there should not be a problem with VIs shearing over time, which can be with wider-viscosity spreads on dino-based oils. The truck is going to continue to be mostly used for towing and given the stupidity in new and more-recent used truck prices, never mind that it runs great and while the paint is somewhat hosed I might actually invest in having it painted.

Thoughts?
 
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In your situation I would use the 75w-140 synthetic only. I've switched all my vehicles to 75w-140 synthetic. A while back SAE rearranged gear oil viscosity which made 75w-90 thinner at the higher temperature scale apparently for fuel economy. That's when several oil manufacturers came out with 75w-110 which covered the original 75w-90 specs.
 
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I have had 04 F150 and currently have 15 F150. I do not tow anything meaningful and I use 75-90 in my lockers. If I towed, I would use 75-90 up front and 75-140 in the rear. From what you stated no brainer IMHO. Any reason not to use 140 over 90? I'm using 75-90 in my 17 Subaru front and rear. Your truck is certainly seeing heavier-duty use than my Sub does!
 
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there are fully synthetics + REAL synthetics!! my 2011 fronty OE is Nissan semi syn 75-140 for my Nissan Dana rear BUT Dana recommends 75-90 REAL synthetic + i use REDLINE one of the FEW REAL synthetics. the 75-90 + all other redline drivetrain lubes netted 3 mpgs!! i haul some coal yearly but otherwise EZE. theres 75-110 Redline as well. a friend had a humming rear + upon draining noted some water in it + he replaced it with Amsoil real synthetic + it got quiet + ran until he traded about 20 years ago, a dirt track friend racer noted with real synthetic his gears stayed a LOT cooler, good luck whatever you choose.
 

tickerguy

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Yeah I'm wondering if I basically toofed the bearings due to running what was once a heavier oil and yet, after the change, wasn't yet still said it was the same as the OE spec on the label......

The bearings DID last 20 years but unless I get a nasty surprise when I get the chunk out and look at it more-closely it appears the ring gear is in perfectly good shape with no chunks or obvious ugliness in the teeth that I can see on casual, still-in-vehicle inspection nor is it misbehaving in terms of any sort of skipping or lock/unlock problems -- but the noise, well, it is what it is and changing the oil didn't do anything so a teardown is obviously next. I suspect once I get the chunk and pinion out it's going to be blindingly obvious both the bearings and races are trashed.

Given that with the trailer on the back if I manage 12mpg I'm doing very, very well (typical is 10-11) whatever small hit might occur in terms of fuel burn is not material, especially when it looks like I just ate $200+ worth of bearings plus my time and trouble to change them, assuming the gears and carrier are ok.

(I've run Redline MTL in my Mazda MTX, which requires a GL4 and its got well north of 200k miles on it without a hint of trouble...... may have to source some of that for the diff, but obviously in the correct grade.)
 
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there are fully synthetics + REAL synthetics!! my 2011 fronty OE is Nissan semi syn 75-140 for my Nissan Dana rear BUT Dana recommends 75-90 REAL synthetic + i use REDLINE one of the FEW REAL synthetics. the 75-90 + all other redline drivetrain lubes netted 3 mpgs!! i haul some coal yearly but otherwise EZE. theres 75-110 Redline as well. a friend had a humming rear + upon draining noted some water in it + he replaced it with Amsoil real synthetic + it got quiet + ran until he traded about 20 years ago, a dirt track friend racer noted with real synthetic his gears stayed a LOT cooler, good luck whatever you choose.

Explain this fully synthetic + real synthetic comment. Lost me there. I'm running Valvoline 75-90 in my F150 and Subaru OB. It's synthetic but what is fully v real synthetic?
 
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There will inevitably be comments about 75W-140 reducing MPG to which I say a headwind or your right foot will impact MPG more than a viscosity change ever will.

I would rather have the extra protection.
Sometimes, particularly with hypoid gear set, higher viscosity can reduce traction coefficient especially if the oil was PAG-based.
 
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I'm not sure how the G80 locker may enter into failing bearings. I had a '08 Silverado 4wd for some years with a regular live rear axle. I used to change out all fluids every 35k-40k as well. As I recall, GM spec'd 80w90 full syn in the front diff and rear. I believe they mentioned not using xxW140. They only approved 75w90 for use in Canada or other severe cold use. I think that was a change from earlier models. That might have been for the front diff only? I did use ST 80/90 syn blend one time with no harm. I didn't tow with that truck.

My '82 Dodge D150 (8 3/8 axle) specs 80W140 only for the live rear diff. Seems like 75W140 is what you can find. More than 40 years ago I had a '67 Jeepster Commando. Between the two diffs, transfer case, manual trans and steering knuckles, Kaiser called for 3 different gear oil weights. That's when I first discovered 75W140.
 
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You're just asking a lot of a 8.5"/8.6" Corporate 10-Bolt, They usually have a mix of Timken & no-name bearings, Usually the carrier side bearings are no-name bearings & go first. Don't be surprised if they've gotten hot & fused themselves to the carrier.....Remove the cage & rollers, Apply some pressure to the Race with a puller, Hit the Race with a Torch & the race will pop off no problem.

Use Timken or KOYO bearings ONLY for the Pinion & Carrier side bearings. KOYO/Torrington ONLY for the Axle Bearings.

Getting the diff set back up correctly is critical, The Pinion Shim & Carrier Side Shims can have wear & need replaced. New Pinion Bearings require a hefty initial preload as does Carrier Side Bearings.
The Pinion Shim will be marked (A 36 denotes a .036" Shim which is critical to get the Pinion Depth right without measuring it)

There are no aftermarket 1 piece Carrier Side Shims like the factory uses, But there are captured type adjustable aftermarket shims that work very well. Individual Side Carrier Shims Do Not work without a Case/Housing Spreader!!

The gear oil viscosity is the least of your concerns right now, Besides.....75w90 didn't likely cause the failure in the first place. However....A 110 or 140 viscosity wouldn't hurt.
 

tickerguy

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That makes sense..... I'm going to be pulling it apart in the next couple of days and we'll see what's what once its apart. Given the noise I'm pretty sure I'll be able to detect roughness in the pinion bearings without removing it if they're bad, particularly given that load isn't required for it to make noise. I wouldn't be surprised to find the pinion bearings with no noise or roughness and the preload within "used" specs -- but the carrier bearings trashed. If so I may just replace those and their respective races, check that lash is in-spec and put it back together.

As I said there was almost no debris on the magnet at all; a bit of silver, but very little and the oil itself looked pretty good so I'm reasonably confident I caught it before things got expensive. We'll see.

I'm definitely not buying Chinese crap for parts; Koyo I'm comfortable with and of course Timken, assuming they're not counterfeit, that is.... :)
 
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I don't worry about mpg or power loss. However, I do somewhat worry about excessive heat from hydrodynamic friction with a 75W-140. If seeking something better, perhaps split the difference with a 75W-110.

I use High Performance Lubricants Differential Life 75W-110 in my 2002 Tahoe that tows a 6,000 lb load pretty regularly.
 
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The bearings DID last 20 years but unless I get a nasty surprise when I get the chunk out and look at it more-closely it appears the ring gear is in perfectly good shape with no chunks or obvious ugliness in the teeth that I can see on casual, still-in-vehicle inspection nor is it misbehaving in terms of any sort of skipping or lock/unlock problems -- but the noise, well, it is what it is and changing the oil didn't do anything so a teardown is obviously next. I suspect once I get the chunk and pinion out it's going to be blindingly obvious both the bearings and races are trashed.
Can you post some pictures?
 

tickerguy

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Once I have it apart, sure.... This is what the ring gear looks like right now.... the lighting is a bit wonky since I'm under the truck with my phone, obviously.

Betcha the bearings don't look like this :)
 

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Explain this fully synthetic + real synthetic comment. Lost me there. I'm running Valvoline 75-90 in my F150 and Subaru OB. It's synthetic but what is fully v real synthetic?

Terms like "full synthetic", "real synthetic", and "100% synthetic" are just marketing terms though some (including in this thread) try to separate them to mean something else. Benjy still thinks it's 1980 so his comment isn't surprising. It's funny that he mentions Amsoil "real synthetic" but last I heard, their SVG oils are more group III than IV. The truth is that group III base oils can perform comparably to group IV/V blends in many areas of oil performance. In fact, group III/III+ often produces stronger HTHS numbers than group IV.

Unless you're operating at sub-zero (°F) temperatures, there's little to no benefit to using group IV over group III. Just increased cost. Group III base oils have advanced substantially in just the last 20 years.
 
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I think this explains better than I can
Not really--it's either conventional or synthetic (from the article):


"an oil is “synthetic” and may be labeled as such provided it uses only Group III, Group IV or Group V base oils in any combination or ratio.

The oil could use 90% Group IV and 10% Group III base oil; or, it could use 90% Group III and 10% Group IV base oils. As long as it only uses Groups III, IV or V base oils, it’s a synthetic oil."
 
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Terms like "full synthetic", "real synthetic", and "100% synthetic" are just marketing terms though some (including in this thread) try to separate them to mean something else. Benjy still thinks it's 1980 so his comment isn't surprising. It's funny that he mentions Amsoil "real synthetic" but last I heard, their SVG oils are more group III than IV. The truth is that group III base oils can perform comparably to group IV/V blends in many areas of oil performance. In fact, group III/III+ often produces stronger HTHS numbers than group IV.

Unless you're operating at sub-zero (°F) temperatures, there's little to no benefit to using group IV over group III. Just increased cost. Group III base oils have advanced substantially in just the last 20 years.

I was hoping to leant something new but that didn't happen!
 
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