The machine shop in Tucson that did the work on my 455 (Larry's engine and Marine) said that, due to the large size of the main journals on the 455 crankshaft, they set the clearances for 20W50 oil. I'm not totally clear on the reason why, but it has to do with getting enough oil in those large diameter bearings. I broke it in with 15W40 Rotella, but immediately switched to Valvoline 20W50. I would be comfortable going back to the 15W40, but think maybe the 20W50 is a better choice for me in Southern AZ, where I am often driving the coach in 100 plus degree temps, at least during the Summer.