Synthetic and regular oil can be mixed. In fact, some vendors sell
this combination as a "blend".

Synthetic is better. It can boil and will still lubricate. I use
it in both our Volvos. I change my own oil so the added cost is
not too great. I mainly use it becuase it offers some advantages
with my turbo engine (the turbo heats the oil a bit more as it
passes over the turbo bearings) and should the engine ever loose
oil pressure (failed pump, broken filter, smashed oil pan, whatever)
then the synthetic will lubricate a bit longer as you pull off the
highway. Yes, that is an extreme reason (and is basically insurance)
but I got justify it somehow :)

If you have a turbo engine then I'd go with the synthetic.