If you are talking about a different brand name but same viscosity, such as a 5w30 blend that is fine. If you are going from a petroleum product to a full synthetic blend it is recommended for a high mileage vehicle to be flushed or a detergent aid added to the new oil.
I would absolutely recommend talking to the people who do your repair work about that. There is no need to worry about mixing oils.
Generally if you are thinking of switching from a non-synthetic to a synthetic oil, the only caveat is that once you switch you really should not switch back.
My general thought is to use what your manufacturer recommends. It is more important that you do your oil changes regularly - no matter *what* oil you are using.
Hope that helps!
180 Degrees Automotive
〉 Answered on Oct 12th, 2009 by Bogi Lateiner, Owner and Technician at 180 Degrees Automotive