Several companies offer Synthetic/Mineral Oil blends, so there is nothing wrong with mixing them. Just like there is nothing “wrong” with mixing brands or viscosities. No more so than drinking a Coors after polishing off all the Bud.
From the Castrol Website:
“Synthetics are sometimes mixed with conventional mineral oils to produce a cost-effective middle ground between the two, referred to as a "semi or part-synthetic" However, while semi or part-synthetics and conventional mineral oils are both capable lubricants, there is no contest as to which type of oil does the best job. For the highest level of engine protection, fully synthetic lubricants come out on top every time.”