regularly. All that is happening is the oil will break down a little bit quicker than if they weren't mixed and the overall oil quality will be diluted. In going from Synthetic to Conventional and vice versa during an oil change, I doubt there is too much residual oil to really make a major difference, especially depending how well you drain. However if you're adding a half quart to a quart of conventional to synthetic regularly or something like that, you're probably not getting the quality of oil you're paying for, and not giving your motor the tender sweet quality oil love it needs.
