Gear oil in axel does it get thinner or thicker?

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Does gear oil in a rear axel get thinner over time as it shears down, or does it get thicker with age as it oxidizes? I have seen both theories stated on this site. Also, FWIW my old (retired now) mechanic said never change gear oil, as it will cause the axel to start to whine. He was no expert on lubes, but had real world experience with servicing vehicles. He saw that when he did real axel oil changes, customer was soon back with a noise complaint. This would support the oil getting thicker over time, and then when new oil (thinner) installed the worn axel now has noise. What do you think???
 
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I don't know in the real world about thinner and thicker. I suppose it has to do with time vs hard usage and heat which way it goes. I do believe in changing gear lube periodically though. FWIW,I changed gear lube in my old '64 VW Beetle and a '69 VW Bus.(integral differential shares lube with the 4 speed gear boxes) that both looked like the lubes had been in there since the Nixon administration. VW used to say the trannys wer "lubed for life" so theyed probably never been changed. I couldn't tell that there was any change in viscosity though. No problems with the lube changes after tens of thousands of miles afterward. They seem to shift easier and if anything run quieter.
 
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I would think it would thin over time due to shearing. The axle is not an open system like an engine so evaporative loss should not be an issue. It also does not encounter the same heat conditions an engine does so oxidation would not seem to be an issue.
 
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 Originally Posted By: oilslick44
I drained the rear axel on a 51 Plymouth in the mid-eighties which oozed out like thick tar over several days. It must of been original.
That would be time winning out over hard use, I think.
 

Bluestream

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I read a post by MolaKule a few years ago, and he suggested that gear oil first shears down and gets thinner, then oxidizes and get thicker over time. Hopefully he can shed some light on this, or correct me if I am mistaken...
 
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Perhaps your mechanic used the wrong lube or didnt use a necessary additive? I changed out gear lube on a vette, camaro, and suzuki sidekick with no whine...
 

Bluestream

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How many miles were on the Vette, Camaro, and Suzuki? I think the right put the right fluid in, and as for the additive; that would cause an LSD to growl and not whine if it was missing. You have done a sample of 3 vehicles with no problems. I am talking about a guy who has done hundreds to thousands of fluid changes over a lifetime career and mentioned to me that it is not uncommon to have a whine develop in a high milage vehicle after a rear axel oil change
 
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The general pattern for engine oils is to shear a bit thinner, then get thicker when they are really beat up and old. Gear oil would seem to be the same. We are talking slight changes over a long time. You old mechanic had bad advice about not changing gear oil, and for the wrong reasons on top of that.
 

Bluestream

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 Originally Posted By: mechtech2
You old mechanic had bad advice about not changing gear oil, and for the wrong reasons on top of that.
I would agree with you, although he never gave a reason, he just stated his experiences. Gear oil should be changed around the 1500-5000 mile range when a vehicle has just broken the axel in. Most people wait until they have racked up huge miles and then change it. FWIW, I once owned a Jaguar XJ6. Changed the gear oil when I bought it, and rear end started to (slight) whine the next day. Other owners on Jaglovers.org had similar experiences when changing their rear end lube
 
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 Originally Posted By: Bluestream
Gear oil should be changed around the 1500-5000 mile range when a vehicle has just broken the axel in. Most people wait until they have racked up huge miles and then change it.
Prolly a good idea. I changed tranny lube ( + both diffies and transfer case) in my truck at about 10K miles from the factory fill to syn.
 
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Hi, Bluestream - I can only relate my experience with Class 8 truck (tandem) drive axles where the lubricants were tracked by UOAs over several years - as follows; Lubricant - 75W-50 GL5 synthetic Supplier - Castrol and Mobil Oil change interval - none - replaced as needed (determined by UOAs) or anticipated at 1m kms (620k miles) Lubricants actually replaced at around 1.2m kms (745k miles) at sale Average operating temperature - 90C front diff Average operating temperature - 94C read diff Aloowed maximum - 120C Maximum reached - 102C The lubricants increasd in viscosity during their service life but did not increase beyond the 20% allowed by the Axle's Manufacturer Contaminants never reached their individual condemnation point(s) and wear metals never reached the Manufactuer's limits shown below; Iron - 1500ppm Copper 250ppm Chromium - 10ppm Aluminium - 150ppm I hope this is of some value With old gear lube formulations your old retired mechanic was probably quite correct in his experience as I recall
 
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I suspect that it depends on the amount of VII in the gear oil. I would think that 75w140 or 85w140 would shear more than thicken, and 80w90 would thicken more than shear. But this is just a guess.
 
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Gears and especially Hypoid or worm gears will sheer down an oil. How much & how fast depends on oil temperature, VI improvers and base stock.
 
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Interesting discussion. Recently, I spent hrs googling for diff whine as I have it in my RAV4. I found countless posts on various forums reporting the same phenomenon as the OP mentioned, i.e. gear oil change at some high milage (usually 100,000 miles or so) followed by whine, sometimes fixed by switching to heavier grade. I had a similar thing in my subaru. I changed front and rear diff oils from the OEM dino SAE90 to redline 75W90 to develop whine in rear diff in 10-20,000 miles. I think problem is dino oil thickens with long use and masks diff wear and whine in old diffs. New gear oils tend to be 75W90 or 80W90 and will initially shear down with use exposing diff wear and noise. This is almost like the AT service thing. Either service early and regularly or not at all.
 
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Originally Posted By: ZZman
I would think it would thin over time due to shearing. The axle is not an open system like an engine so evaporative loss should not be an issue. It also does not encounter the same heat conditions an engine does so oxidation would not seem to be an issue.
I agree. My father's GN has a temp probe in the rear end and temps rarely go 50F above ambient. In fact, they're usually within 20 degrees of ambient with normal driving. To be fair, this car only saw city driving, no freeway driving.
 
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Originally Posted By: friendly_jacek
Interesting discussion. Recently, I spent hrs googling for diff whine as I have it in my RAV4. I found countless posts on various forums reporting the same phenomenon as the OP mentioned, i.e. gear oil change at some high milage (usually 100,000 miles or so) followed by whine, sometimes fixed by switching to heavier grade. I had a similar thing in my subaru. I changed front and rear diff oils from the OEM dino SAE90 to redline 75W90 to develop whine in rear diff in 10-20,000 miles. I think problem is dino oil thickens with long use and masks diff wear and whine in old diffs. New gear oils tend to be 75W90 or 80W90 and will initially shear down with use exposing diff wear and noise. This is almost like the AT service thing. Either service early and regularly or not at all.
Does the RAV4 diff share fluid with the trans? Most TLs have some whine but mine was 75% cured by going with the thicker of the Amsoil fluids, ATD. It's literally quieter now at 78,000 miles than it was when I first took the factory fill out at 30,000.
 
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Originally Posted By: BuickGN
Does the RAV4 diff share fluid with the trans? Most TLs have some whine but mine was 75% cured by going with the thicker of the Amsoil fluids, ATD. It's literally quieter now at 78,000 miles than it was when I first took the factory fill out at 30,000.
Yes, it does. I did the same thing. Dumped Toyota's WS and filled with thicker Redline D4 at 30,000 miles (today). Did not test yet if it helped but would be surprised if it didn't.
 
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I wonder if it's possible to become "non-existent" after a period of time. On my late 60's Chevy pickup bed trailer with the 12 bolt I just added almost 2 quarts of lube. From what I gather that's how it should take empty. I didn't open the pumpkin first. I've used this trailer almost 2k since I got and who knows before that. Granted it's a trailer so maybe it doesn't need it. On the side note I bet it will self-destruct tomorrow on a 200 mile trip.. that's my luck.
 
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