75W-140 Synthetic In The Rear Differential All The Time

[NORVIN]

I have a 2003 KJ Limited with 84,000 miles. One of the rear wheel bearings needs replaced so a mechanic will be taking the rear apart to fix that. I am wondering about this in the owner's manual:

Axle Differential (front-rear)
Mopar SAE 75W-140 (API GL5) Synthetic Gear Lubricant or equivalent.
The 8.25 Corporate Rear Axle should use a SAE 75W-90 Gear Lubricant.
For trailer towing, the lubricant should be replaced with SAE 75W-140 Synthetic Gear Lubricant. Models equipped with Trac-Lok™ require a friction modifier additive.

I want to be able to do some towing as needed without worrying about draining the rear axle of 75W-90 and replacing it with 75W-140 and then draining that out when I am done towing to put 75W-90 back in.

Is there any reason to not run 75W-140 all the time?

 

[turblediesel]

Very cold weather is the only reason I can think of.

 

[NORVIN]

Very cold weather is the only reason I can think of.

Is it though? 75W-90 and 75W-140 should flow the same when cold since both are 75W. It's when it's hot that it is different, right?

When I look at the PI Sheets for Valvoline Full Synthetic 75W-90 and 75W-140, they both have a pour point of -48°C. At 40°C, 90 cSt is 92.5 and 140 is 184. At 100°C, 90 cSt is 15.6 and 140 is 27.

My understanding of it from the above is basically the thicker sticks to the gears, bearings, and axles better when it's hot and under a load from towing.

 

[turblediesel]

That's more scientific thought than I've ever given it.

Maybe it's a MPG choice?

 

[ltd02]

That's more scientific thought than I've ever given it.

Maybe it's a MPG choice?

Yep. MPG thing. I ran the 140 in both mine as long as I had them.

 

[mandala1111]

The reason for the 75W-140 is for heavier use and also advised if you are in a warmer climate. The higher number means a thicker viscosity at running temp's.