Lets say gear oil in this case...can I mix 80w90 with 85w140 with no ill effects?
I hope so.
I've just got doing that to get rid of a whine in my 2007 rear diff using Amsoil 75-90 gear oil.
About 1/2 of a quart of the 145wt stuff quieted it right down.
It is safe. Try to stay within the same brand though. Esp with motor oils.