Done that before 1-2 times for my car and superbike. Only mixing small quantity, like 1 L into 4 L. Don't see, nor feel anything to the engine sound, performance, fuel consumption. When change oil 3 months or 6 months later, don't see any different in oil color either.
I read from engine oil specialize homepage, which touch about the topic a lot. Most of them said not recommended. I believe it should be "not recommended for long term use"
I mixed different brand, but same grade though.
Even for brake fluid, I mix with different brand, and same grade. Flushed it, and all with same brand, same grade. No different is brake control and feel either. Again, I believe this is not for excessive use