I don't break old Mopar engines in with synthetic, as I dont' feel the rings can seat properly. My experience with the 383 was that the tolerances in the 69' engine were much looser, which contributed to the poor running.(piston to skirt clearance, bearings,etc) The side symptom was I had an oil leak behind the oil pump that I couldn't stop, it was incessant! Once I went to Rotella Diesel with zinc, the oil pump quit leaking, and my oil pressure stabilized at 55 psi, the lifters turned silent, and it pulls aggressively from 40-110 mph. (I left the 2:76 ratio in the car)\It has been seven years, and around 5,000 miles. The engine only runs better and better as the miles get put on. Others may have a different view, but I had the same experience with my 440, and 340 dart. I finally got it. Now, some here like Royal blue, so my next engine, another 340 will get that after the break-in period. As long as it has the features similar to the 60's oils and Zinc above 2,000ppm, I will try it.