Nothing, it'll be fine. If you read the fine print, it'll usually state that it's blendable.
Nothing happens, they just mix. You can buy "blended" motor oil.
Oil treatments is a total waste of money so don't mix anything with the synthetic oil.
Yes, you can. It is even sold as a Synthetic Blend.
Yes.
It might be... It's highly not recommended to mix regular and synthetic oils.
Yes
Yes, you can even mix them.
Yes, they even sell it that way.
Oil changes are always necessary. If you mean can you mix conventional oil and synthetic, the answer is yes you can.
Yes, you can even mix synthetic with conventional oil.
Synthetic and conventional oil can be mixed and used with no problem. That is exactly what a synthetic blend that you can buy is. A mix of synthetic oil and conventional.
use it, it is ok to mix