I own 4 GMC motorhomes and our net group has been swatting this problem around for a few months. One of our vendors has "lost" 4 new rebuild 455 engines due to poor protection provided by the new oil formulations. This was documented by Jasper Engines - the performance side- not the stock rebuild side- as they ponied up to replace the engines on their dime. These failures were all cam and lifter related on the new engines due to wear. They determined that is was the very low ZDDP concentrations in the new formulations.
So, cutting to the chase the vendor and Jasper have found an off the shelf solution with a Valvoline dino oil product. It is the Valvoline VR1 racing oil. It has 1200 PPM zinc and 1300 PPM phosphorous. It meets all other requirements and has the new SM rating as well. Documented direct from Valvoline. They also have another one but is does not have any detergent (or very little) and is not recommended for street use. I know it comes in 50W and not sure if they have different weights also.
This is probably not the last word on this problem but it's what I know and I trust the man that did the search. He has more to lose than most as he puts in 15 to 20 engine s a year and has to guarantee them.
Here is the sight :
Daily Pose
It changes on a daily basis so it will be in the archive are in about a week.
Marcus