You might want to read this:
Ok here's the official scoop on the oil issue. As I said earlier, I asked the highest people on the planet when it comes to rotaries. First I'll start with Rick Engman's position as he built all of the engine's for Downing. I looked at Rick and just quite simply said, "synthetic oils". His response was, "I wouldn't use anything but synthetic oil in a rotary". Thankyou Mr. Engman! All of Downing's race cars use Valvoline synthetic. He said it doesn't matter if it is a race engine or not. I also told Mr. Engman that many people say you only use synthetics in a rotary because it gets rebult all the time and that it doesn't matter what happens to these engines. I can't post what his opinion are of these people! He did say their engines get 100 RACE hours of use between rebuilds. The Courage is using experimental engine parts that were last used in the 80's and 90's! They care more about the longevity of the engine on a race car than a street car so some people's logic is VERY flawwed.
Now onto Mazda's position on synthetics since I know that the opinion of the smartest rotary person in the U.S. isn't good enough for some people. As I stated earlier, I asked the head of rotary engine development for the past 30 years at Mazda. He designed and built the 787B engine too. In very broken english he told me that not all synthetics are created equal. some are better than others when it comes to mixing with fuel in the oil metering system. The oil metering system is the key. It is NOT with swelling seals. He said that because some oils work very well and others not as well, it is far easier to just officially say not to use them than to say which ones to use and not to. This has actually been speculated for a long time and now we know it's true. I asked which synthetic oils he does not like. the answer is very surprising. The worst synthetic oil for the rotary according to the smartest rotary engine person in the world is Mobil 1. At least Valvoline and Royal Purple are fine. Didn't get any others listed. Rick Engman did say that decades ago there was an issue with some swelling of seals with old synthetic formulations but these have long since been resolved.
To all the people who keep getting on my case and others for using Royal Purple, WE'VE BEEN THE CORRECT ONE'S!!!!!!!!!!!!!!!!!!!! 
source: RX8Club.com - View Single Post - Synthetic oil - Another one
