Wingboat:
You can also go to
www.mobil1.com. They have a FAQ's about this subject.
This is an exact quote off the web site:
"Can different synthetic motor oils be mixed together?
Mobil 1 is fully compatible with conventional motor oils, semi-synthetic motor oils and other synthetic motor oils, should it be necessary to mix them. However, it is important to note that the superior performance of Mobil 1 will be reduced by diluting it in any way. "