Mixing the oils isn't a problem. It's what happens with some of the seals (depending on material) when switching back to dino. While it may be coincidence, I've seen enough cars sprout little leaks here and there after the change over to question it.

As for oil compatibility, this is a nonissue. Some companies like Valvoline sell 'blended' oil that are half synthetic and half dino. Why anyone would want this, I don't know, but....