Mazda doesn't recommend synthetics for the Wankel engines in the RX-8, but it has nothing to do with the O2 sensors. It has everything to do with the rotor tip seals. Wankel engines deliberately burn a small amount of engine oil by pumping it to the rotor apex seals to keep them lubricated. Synthetics leave deposits that apparently will eventually cause the apex seals to fail. I don't know if this applies to the group III+ synthetics or not (I doubt that it does, its probably just PAOs) but if I had an RX-8, I wouldn't push my luck!