I use conventional Castrol 10W30 as recommended by the machine shop and my father. I change the
oil every 1000 miles so its not realistic to use
synthetic. If I was going to road race this engine in high temp and extreme condition I would probably use
synthetic. My Wife's car and my every day drive get mobil 1
synthetic just because I change it every 6000 miles. If your motor runs good on Royal Purple and doesn't burn
oil I wouldn't change it but thats just my opinion.