Oils will readily mix. The only oils that don’t mix are Poly Glycol Synthetics, and they typically go no where near a car engine. Some people have a vested interest in perpetuating myths about oils.
Is it worth the risk though? I learned the hard way about mixing oils when I topped up a machine and it seized up a week later. The oil had turned to a thick jelly and wasn’t much good for the splash fed gear lubrication. The warranty cost me 20k.