As long as it's the same brand and mixture (ie 5w30 and 5w40 Pennzoil Platinum) it's fine. Mixing other brands or blends may introduce competeing additives which can cause many problems (thickening, acidity, and so on).
Mixing your oil grades isn't recommended though unless you premix them for a certain target viscosity and always fill up with that (ie 2 bottles or 5w30 and 1 bottle of 5w40, premixed and that's what you always use... this doesn't mean that when low you just top off with either).
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