Switching/Mixing Motor Oils?

Hello,

I ususally get synthetic oil. Sometimes however, while travelling, I
need to top up, and put in a regular oil.

In the past, the mechanic would give me an oil change for free - but
it would be regular oil (no synthetic). Other times if synthetic was
handy it would go in.

Is there a problem with mixing or switching motor oils? I have a 92
Passat with high mileage.

Thanks,

Tmuld.