HelpWrong Wt. Oil In Differential?

Messages
7
Location
Hawaii
I just aquired a 95 Subaru Legacy 2 wheel dr. and the differential was about over a half quart low on oil. I drained out the old oil and replaced with Valvoline 75-140 GL5 gear oil. Is it ok to run with this heavier wt. oil? The manual recomends 80-90 wt. but the differential is so much quieter with the heaver oil. Is it true that the mileage will drop using the heavier oil?
 
Messages
210
Location
SoCal
I doubt there's a problem with that, especially in HI. You might get 0.x less MPGs and 90 weight gear oil in 1995 is more like 110 weight today.
 
Messages
4,998
Location
Milwaukee, WI
For sure the mileage will drop. I'm not sure it will drop enough to cost more than the four bucks to buy a new quart of differential oil. Honestly, it sounds like you have a leak, so another differential oil change may be in your future when you change the gasket.
 
Messages
19,479
Location
Chicago Area
It is warm enough [consistently] where you live so that it is not a big issue. Technically, it can cause a small MPG drop. You like how quiet it is? I'd leave it in, then.
 
Messages
39,806
Location
Pottstown, PA
If you're not feeling the difference, I don't know if I'd worry about it. The impact on fuel economy will (more or less) depend on trip length. The closest you're going to get to the same visc is not too far off of double 75w-90 @ 100C (something like 15Cst vs 25 Cst). At worst case, you're simulating 75w-90 in colder temps over a longer duration. As was mentioned, it may not amount to much, but YMMV.
 
Messages
8,756
Location
RI
If it works, leave it there. Your climate isn't cold enough to worry about it. And, I'd bet that a 75w140 would shear into a 110wt soon enough. Keep it topped off and change it again in 30k.
 
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