My Keith Craft stroker came with instructions that recommend using non-synthetic 10w40 oil. I have been using Pennzoil 10w40, but after I reach 3000 miles will switch to Mobil 1 10w30 synthetic oil.
Using a synthetic oil during the break-in period may not allow the piston rings to seat properly.
i use 20w50 castrol in mine. my engine still isn't totally broken in yet as it only has driveway miles on it, i will eventually switch to a synthetic either mobil 1 or valvoline. i have used castrol syntec before and i wiped 2 lobes off the cam after i switched to it, but that is more likely because it was when it first came out and they only had 5w50 at that time.