Keep in mind, every time you change the oil you leave roughly 5-15% or about 1/2 to 1 full quart of old (often unknown) oil in the crank, pan (and filter if you don't also change filter). By virtue of math, that old oil is a small percentage of oils from many oil changes ago.
You are almost certainly "mixing" oils of various brands and sometimes even weights (unless you know for certain the exact same brand/weight and formula has always been used).
Yeah, it's generally going to be fine b/c makers know oils get mixed routinely.