Mobil1 75w-90 vs brandx 85w-140

[oil pan 4]

Brandx is whatever I can get the cheapest.
I have my beater 1999 dodge Dakota 5.2L with 9.25'' rear, just replaced the pinion seal, today like an hour ago.
I was going to filler up with Mobil1 75w-90 because:
9.25'' is plenty of gear for a half ton pickup.
I already have it, mobil1 75w-90.
Between the tractor, brush shredder and this leaky dodge rear end most of my 140wt is all used up.
M1 has great, if not the best oxidation stability.
M1 doesn't have the best shear stability so I'll just top off once a year with 140wt.
The truck normally doesn't get driven a lot.
I usually don't do any heavy towing.

But tomorrow I'm going to go to my friendly local coal fired power plant, 40 miles away (down wind) and get a bed full of coal.
How much coal can a standard bed dodge Dakota hold? I don't know but I'm fixing to find out.
The rear diff is mostly filled with 75w-90, I didn't drain it completely out when I did the seal.
Usually the truck never gets driven more than 20 miles at a time, rarely more than 50 miles.
I'm going to test the temperature with my probe thermometer and it will probably be about 100f tomorrow.
At what operating temperature does a 140wt oil start to become a good idea?

 

[slacktide_bitog]

If you already have it, use it, especially if you already have 75w90 in there.

 

[dnewton3]

For the limited application you speak of, it won't matter one way or another.
Mobil 1 is likely to be able to stay in service longer, but given the inability of the average BITOGer to keep hands off the equipment, that won't matter either.

 

[oil pan 4]

For the limited application you speak of, it won't matter one way or another.
Mobil 1 is likely to be able to stay in service longer, but given the inability of the average BITOGer to keep hands off the equipment, that won't matter either.

I'm going to try and leave it alone for about 5 years or 40,000 miles as long as it doesn't leak any more.

 

[oil pan 4]

It's 90f right now. Drive for about 30 miles at 65mph.
180f gear oil.
The rear is still leaking a bit so I got a new rubber fill plug and stuck it in there after I took the truck rear diff oil temp.
180f.
Could have been as high as 185 while on the highway. I drove the last 4 miles around 40mph.

 

[oil pan 4]

Leak is stopped but I struck out at the coal fired power plant.
Ya can take all the flyash you can carry.

 

[oil pan 4]

Towed 3,000lb about 40 miles in 90F heat, when I got home the diff temperature was 190f. I never would have guessed adding 3,000lb would have only added +10f to the rear diff temp. Drove about the last 4 miles at 50mph so it could have seen 195f on the highway.
Definitely not seeing any need to go to a 140wt except for leaks and top offs at this point.
9.25'' is plenty of rear diff for a half ton.