Mixing Gear Oil Viscosities?

[Matt42]

What awful things will happen if I mix gear oil viscosities? In this particular situation, I am looking at mixing in a quart of Valvoline synthetic 75W-140 with a couple of quarts of Valvoline synthetic 75W-90. Same brand and type. The mix would be in the Dana 80 differential of my 1996 2500 with manual transmission.

If I recall correctly, the truck can use either weight.

 

[Ozymandias]

Dana 80 runs on salad oil just fine, so nothing will happen.

No seriously, you are fine that way as a modern 90 weight oil is far superior over a 30 year old 140 weight oil.
I give it a thumbs up.

 

[Matt42]

Well, a 1967-ish Rolls Royce reportedly got a punctured oil pan in the tropics a long time ago. They plugged the hole and drive it back to a place for repair using banana oil. So I am not surprised.

 

[Ozymandias]

 

[Diesel Gunner]

A slight reduction in viscosity by blending 90W with 140W. Just like mixing some 100 octane with 91 octane in a fuel tank. Being that they are both Valvoline synthetic the additive packs are probably nearly the same, if not exactly. If they were different brands there is probably not much of a chance of anything catastrophic happening, but I always advised against it. After all, how much trouble is it to pick up a quart of matching oil next time you are in town?