here's something interesting on the use of synthetic oil
"The voluntary recall of all 2004 and 2005 vehicles, and some 2006s, is expected to be announced this week or next. It involves damage to the catalyst resulting from oil leaks in the RX-8's rotary engine.
Any engine that does not pass a vacuum test must be replaced, said Robert Davis, head of product development and quality at Mazda North American Operations.
Engines prone to failing the test are mostly in hot climates and use synthetic oils."