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?