RX-8 engine failures

I have a 2004 Mazda RX-8 that has had some curious problems in the past. With less than 40K mile, I was hearing a pinging sound, had to have the cat converter changed, irregular acceleration, etc. After many trips to the shop, Mazda determined that it was the result of carbon build-up and replaced the engine. Searching other forums, I found many concerned owners who have experience similar problems. It seems the RX-8 has a serious problem and could use a Rx from the Car Doctors. The general concensus is to switch to 5W-30 oil. Just wondering if anyone else is having luck solving a similar problem. This consumer can’t afford to replace the engine again in 30K miles and has to do something. I would prefer not to sell the car though.