Tuff Torq did something similar. For a while they were shipping with 10W-30 motor oil, and then recommending 5W50 synthetic for their change outs. Then a year or so ago they came up with their own secret sauce.
I'm going to keep on using what I can get locally and for a good price, which in my case was Mobil1 5W50.
I've also used Delvac 15W40 and it worked fine. It was a little touchy when cold, but once it warmed up it ran just fine.