I'm running Castrol Edge (full synth) in all my cars (daughter's civic, wife's CRV and the S). All of them have over 150K miles on the clock. Last spring, my daughter ran the Civic with a stuck thermostat for a couple days. (Her friend's picture was taped over the temp gauge!). I replaced the thermostat, and it has been fine ever since, no blown head gasket, no rod knock, nothing. Full synth is the way to go. The AP-1 does burn off a couple quarts between changes (6-7000 mi), but the piece of mind is worth it.