From the Mobil 1 web page:

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

All of the other synthetic oil brand web pages agree that the oils can be mixed. They do warn that any additive ingredents may be compromised by mixing oils....duh.

Not mixing synthetics with dino based oils is an old adage from when synthetics first were introduced.