4th Gen Rear Diff Fluid Discrepancy?

Wild one

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To start off I live in a small town where everyone knows everyone, I am good friends with the salesman whom sold me the truck and he is great friends with the owner. I never get up sold or even try to, everyone at the dealership knows me etc.



I hit 100,000km and my rear diff fluid looks new and I wanted to change it, I went and purchased 75w140 as that’s what the manual states to use, I went over to visit the salesman (as he is a friend of ours) and my child poses for dealership pictures when the new owners don’t want to, it started as a joke and has taken off he asked if I called the dealer to check the price, I stated no because the dealer is always more expensive. He talked with the parts manager and they ran my vin, as per my vin my rear takes 75w90 and if my truck was in getting work done and they needed to add fluid it would be 75w90. So this got me to digging, the same owner owns a tirecraft (a tire shop that also does mechanic work) and has their own software for instructions on fluids, mechanical work, the time jobs should take and the amounts of fluids needed in all different makes and models. According to their system by inputting my make and model it tells them to use 75w90 and 2.2 litres.



Now I am confused, the manual calls for 75w140 but all other places I’ve contacted including the dealer call for 75w90 I don’t have the limited slip and just the “conventional rear end” 3.21 gears which fluid should I use? Or just leave it alone?
 

HEMIMANN

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They used to call for 75W-140 in the cars before 2010,then between 2010 and 2014 they called for 75W-140 in the V6 and 5.7 cars,but spec'd 75W-90 in the 6.4 cars,then in 2015 they started spec'ing 75W-85 in the cars,and that's when guys started yelling about the diffs howling in the cars,so you can draw your own conculisions,lol

I'd never heard of a 140 gear oil spec in a light duty vehicle until I came over from Chevy to Mopar. Car companies don't make their own axles anymore, so what gives?

140 always and only went into the heavy duty trucks. And bulldozers, and such.
 

retired

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the 75W is for winter compatibility so either 75W90 and 75W140 should be the same in cold weather. 140 gives a bit better shock absorption vs 85 or 90. If a lot of miles or towing I would run 75W140 I doubt the average person will not see any real mpg difference maybe 0.2 mpg at most.
 

Andersoncma

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2011 Ram 2500 6.7L Cummins. I don’t do any heavy towing, and run valvoline Full Syn 75W90. 137K miles, No problems, no issues, removed Diff cover last spring and everything looked brand new and all tolerances appeared to be factory spec
 
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leafsby2

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2011 Ram 2500 6.7L Cummins. I don’t do any heavy towing, and run valvoline Full Syn 75W90. 137K miles, No problems, no issues, removed Diff cover last spring and everything looked brand new and all tolerances appeared to be factory spec
I do tow a 6,500Lbs 36’ camper, don’t go too far though.
 

Wild one

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I'd never heard of a 140 gear oil spec in a light duty vehicle until I came over from Chevy to Mopar. Car companies don't make their own axles anymore, so what gives?

140 always and only went into the heavy duty trucks. And bulldozers, and such.
Alot of the earlier trucks spec'd straight 90 weight hypoid gear fluid,which is probably about the equilevent of a multi-viscosity 75W-140 now.Up till i think late 2017 they spec'd 75W-140 in the 1500's in the owners manuals.
Ford spec 's 75W-140 in their 05+ diffs.

 
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CheechDogg.0n37s

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After my truck go re-geared, the shop that did it said to only go with 4.5 quarts of 85-140 because of the 4.88 gears and the larger and heavier wheel and tire combo ... does that sound right ?
 

HEMIMANN

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This makes me wonder if they've cost-reduced the gears to marginal capacity.
 

zogg

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I just changed both differentials on my 2019 classic. I used Royal Purple 75/90 in both. My manual calls for GL5 and 75/90.
 
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