Synthetic engine oil and regular engine oil can safely be mixed together and nothing bad will happen to your engine when you do mix synthetic oil with regular conventional oil.
However when you do mix synthetic oil with regular oil you lose the benefits of the synthetic oil.
So if you have synthetic oil in your vehicles engine you should keep some extra synthetic oil with you so you can add more synthetic oil to your vehicles engine if you need too.
If your engine is high mileage then you'll want to use synthetic oil without mixing it with regular oil to keep the benefits of the synthetic engine oil.
But if you need to mix some regular oil with your synthetic oil then that's okay and your engine will not destruct or blow up as a result of the mixing of the two oils.
Synthetic and regular oil can safely be mixed.