When you mix regular oil with full synthetic oil, the regular oil will dilute the benefits of the synthetic oil and can reduce the full synthetic oils performance and protection.
However when you do mix the regular oil with full synthetic oil, mixing the two oils will not harm your engine.
Your engine will still be lubricated properly but the benefits of the full synthetic oil are reduced when you mix both regular oil with full synthetic oil.
I've had to mix regular oil with full synthetic oil several times when I ran out of full synthetic oil and needed to add oil to my engine.
Nothing bad happened to the engine and the engine is still running fine.
It's best to avoid mixing the regular oil with full synthetic if you can, but it won't do any harm to the engine if you do mix the oils together.
The only thing that happens if you mix the two types of oils is that the regular oil or conventional oil keeps the synthetic oil from performing to it's full potential.
Also it's best to not go back to regular oil after you start using full synthetic oil although you can if you need too.
Synthetic oil has smaller molecules and higher densities.
So if you switch from synthetic oil to conventional oil, any gaskets and seals which had no cracks with the other larger molecules, could begin to deteriorate with expansion and heat when you use synthetic oil.