This has been discussed a lot, and I really don't think it's a big deal using it in OPE. I have to admit I feel better with the cert, but I really think, such as the case with some of the Rotella oils, they simply don't want to pay for the testing for the cert for gas engines when there's tons of oils out there to choose from for that cert.
I use 15w40 oil in my gas tractor, now for a long time, and it's a great choice.