This is from the faq page on Mobil 1's site:
Q: Can different synthetic motor oils be mixed together?
A: Mobil 1 with SuperSyn- is fully compatible with conventional motor oils, semi-synthetic motor oils and other synthetic motor oils, should it be necessary to mix them. But the superior performance of Mobil 1 with SuperSyn- will be reduced by diluting it.
And this from the Myths about synthetics page on the same site:
Myth: I need to flush my engine before switching to Mobil 1.
Reality: No special preparation is necessary when switching from conventional motor oil to Mobil 1 with SuperSyn-. http://www.mobil1.com/index.jsp