RE: the OFFICIAL R-40 thread !
The O.S. would be my pick, simply because I've had several, all good engines. I'm looking into a Sirio, a team mate has one, loves it, and it's very fast. I've also been looking into the new O.S. R-Spec 12. It's ROAR compliant, but our track isn't limited, it's run watcha brung, hence the Sirio.