Most rear ends are GL-5 whereas transmissions and transfer cases that use gear oil usually are GL-4.
The ?W140 may be overkill in most autos.
The ?W140 may be overkill in most autos.
what determines what gear oil weight to use in a rear end differential?
- Thread starter Spector
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I too have been wondering about the use of rear gear lube. I have heard that the older Ford F250s called for 80w90 but that when Ford decided to make synthetic a "factor fill" they went with a 75w140.
I wonder why they made the jump to 75w140 instead of just calling for a synthetic GL-5? That would have allowed for 75w90 and 80w90 synthetic which would have gotten much better fuel economy.
For what it is worth, I changed out the dino in the front diff on my 2000 diesel excursion and also the factory fill 75w140 synthetic and replaced them with 75w90 Series 2000 Amsoil.
I noticed a large increase in highway fuel economy. From 19.5 to 21.6 mpg at 65 mph. This increase is attributed to the rear axle gear lube as I did the changes months apart and did not notice any change in economy with the front diff oil changeout.
If Ford wants to get their truck CAFE mileage up they should add 75w90 synthetic as a factory fill and specify changeout to 75w140 synthetic for those who pull heavy duty max capacity service.
Mike
I wonder why they made the jump to 75w140 instead of just calling for a synthetic GL-5? That would have allowed for 75w90 and 80w90 synthetic which would have gotten much better fuel economy.
For what it is worth, I changed out the dino in the front diff on my 2000 diesel excursion and also the factory fill 75w140 synthetic and replaced them with 75w90 Series 2000 Amsoil.
I noticed a large increase in highway fuel economy. From 19.5 to 21.6 mpg at 65 mph. This increase is attributed to the rear axle gear lube as I did the changes months apart and did not notice any change in economy with the front diff oil changeout.
If Ford wants to get their truck CAFE mileage up they should add 75w90 synthetic as a factory fill and specify changeout to 75w140 synthetic for those who pull heavy duty max capacity service.
Mike
I think a thinner synthetic such as 75/80W-90 hurts the lockup performance of posi differentials - this is just a theory.
My open differential used to be quite good in the snow w/ dino 80W-90. And just recently I drove around the block w/ Redline 75W-90, altho it wasn't warmed up yet, and I found both of my tires were spinning in the turns, and this helped grab onto the road better.
Once the rear fluid heats up, I find that i can easily spin one tire only. Again its just a theory.
I'll be using 80W-140 in the E-250's Dana 60 rear end (traction-lok 3.73s) and 75W-90 in the Crown Vic's 8.8" w/ Detroit TrueTrac LSD. All Redline synthetic.
My open differential used to be quite good in the snow w/ dino 80W-90. And just recently I drove around the block w/ Redline 75W-90, altho it wasn't warmed up yet, and I found both of my tires were spinning in the turns, and this helped grab onto the road better.
Once the rear fluid heats up, I find that i can easily spin one tire only. Again its just a theory.
I'll be using 80W-140 in the E-250's Dana 60 rear end (traction-lok 3.73s) and 75W-90 in the Crown Vic's 8.8" w/ Detroit TrueTrac LSD. All Redline synthetic.
MolaKule
Staff member
"What determines the oil weight/viscosity required by a rear end?"
The amount of torque (HP) expected to be transmitted, the temperature of operation, gear type and construction, such as size of drive pinion and ring gear, lockup, no lockup, etc.
The amount of torque (HP) expected to be transmitted, the temperature of operation, gear type and construction, such as size of drive pinion and ring gear, lockup, no lockup, etc.
What determines the oil weight/viscosity required by a rear end? I know most call for 75w90. But on store shelves I see a lot of 85w-140. When would you "need" the Xw-140 oil rather than a Xw-90 oil? Will a 75w-90 oil work in everything?
On my '89 F250 diesel, it has a C6 rear end. I'm not sure if it's a limited-slip or open differential. I do know the cover is 12 bolt. What weight would be best in there? How about the front differential, it's a Dana50?
On my '89 F250 diesel, it has a C6 rear end. I'm not sure if it's a limited-slip or open differential. I do know the cover is 12 bolt. What weight would be best in there? How about the front differential, it's a Dana50?
The best place to start certainly would be at the dealership or your manual. As you say most applications call for 75w-90 but there are API GL-4 and GL-5 requirements to consider also. I wouldn't think an 85/140 would apply to most vehicle applications but only to the heavier duty ones. It is important though to desern the use of a limited slip unit or not, as a friction modifier is usually required for those.
350 lb-ft torque
temperature: -15F to 100F ambient
Gear type: hypoid gears
Ring gear: 9.75" diameter
Limited Slip: Yes, Spicer Traction-Lok
I chose Redline 75W-140 for this, OEM calls for SAE 90 dino.
310 lb-ft torque
temperature: -15F to 100F ambient
Gear type: hypoid gears
Ring gear: 8.8" diameter
Limited Slip: Yes, Detroit TrueTrac (torsen)
I chose Redline 75W-90 for this, OEM calls for 80W-90 GL-5 dino.
Do you see any flaws in my choices, do you have any recommendations? I figure the 75W-140 would protect the gears and bearings while providing better lockup.
The first rear end is a Dana 60 that belongs to a Ford E-250, and the second is a Ford 8.8" from a Ford Crown Vic.
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