As others have commented, the only reason not to use RP is cost.
This engine calls for a twenty grade, no?
Walmart clearanced this grade some time ago for cheap and has apparently brought it back. I've also seen some RP on clearance for two bucks a quart at some AZ locations.
My point is that if RP is an itch you'd like to scratch, you might at least wait for a decent deal on the stuff.
At regular RP prices, there are a host of arguably superior boutique oils to try, like Amsoil SSO or Red Line as well as a number of Euro and Japanese imported oils.