[Injunjoe]

On my first 455 I did the break in with 10w-30 after that I switched to Phillips 66 100% Synthetic 5w-50 The biggest advantages I saw were extended oil change intervals, and the increased temperature operating range of a synthetic. Another very cool point was the fact that after 40,000 miles I tore the engine down (Due to an intake valve failure, Bad valve guides, long story) the interior of the block was spotless. The bearings were almost perfect, only slight scoring from the valve failure. This motor had made approx. 300 passes down the 1/8 mile, at 3830 lbs running mid 8 second passes, 3.08 gear, and the rest of the miles on it were fairly hard street driving (Cant let the mustangs talk too much trash). I was very impressed with the performance of the oil. 20 psi of oil pressure hot idling, and about 52 psi crusing on the interstate. The one single disadvantage I have found with synthetic oils is that if you have even the slightest chance of a gasket or seal leaking the synthetic will wick through and leak. If your motor is nice ant tight though, it is well worth the investment in my book.