now if racing rotaries are running premix - that advocates the point that they have their oil injection systems disabled. With this setup you can use synthetics as it does not have any part of burning and lubricating the apex seals.
As I said - you can run synthetics if you disable the oil injection pump and mix two stroke oil with your petrol. A couple of my freinds have RX8s, one all original TT RX7 and an oldschool 1.2 litre carby RX7 - all cars go to the petrol station with 1 or 2 bottles of 2 stroke in hand. The cars have never had any issues. The carby RX7 has been to Lahore and back (and not on the back of a truck). All these cars still use the flaky oil injection system. But it pays to be safe than sorry