Mixing two different brands of differential/gear oils of same weight
I am thinking of flushing the rear differential oil as I do not know if it has ever been flushed in the past. The odometer reads 122,xxx miles. Better late than never. So, I purchased a quart bottle of a gear oil, Redline 75W90. Shortly after I found out that I need 1 liter of this stuff. I can't find the same brand around my town, but there are other brands with the same weight.
The plan was to replace tranny and differential on the same day. Has anyone mixed two different brands of gear oils with same weights? I don't see why this couldn't be done as most of the gear oils are compatible to others. Any inputs are welcome.