I just didn't like the purple dye in R.P....I like to be able to judge the cleanliness of my oil from looking at it and the dark color makes that a lil more difficult... I also read somewhere online that RP contains a chemical that at higher temps is a corrosive... don't know how true that is or at what temp it becomes corrosive.
One of my friends used to be real big in the racing scene. He always said that RP was a great "one day" racing oil but that you always wanted to change it once you got home. I don't remember the exact reason but this might be it. I once bought a bunch of RP gear lube to change out differentials and read some reviews once I got home. It didn't rate so well based on some of the tests so I ended up returning unopened it and buying another synthetic that was both cheaper and got better ratings.
I don't remember my exact reasoning on this either as it was a while ago.
One thing nice about oil sold for small engines is that they will contain higher levels of anti-wear additives such as zinc and phosphorous that have been reduced in emissions compliant engines. The same also applies to diesel rated oils but to a lesser extent. This is one reason why I run the Rotella. I also ran an ATV/motorcycle oil in a mower once.