ORIGINAL: DRAKE-ULA
wd40 is good stuff, but not the best for after run oil, although it is better than nothing.
marvel mystery oil is a great after run oil. probably as good as any after run oil on the market.
twomanytoys is correct. there is an additive in sidewinder fuels that practically eliminates the need for any afterun oil. i switched from trinity race blend to sidewinder a few years back and the stuff is great. it tunes like the premium fuels such as byrons and odonnels which is really saying a lot.
When you rebuild a 13b motor for an mazda rx-7 which is a rotor engine with an aluminum houseing and chrome sleeves inside they tell you to use marvel mystery oil and tranny fluid on the motor for a few reasons. If any of you ever own an older rx-7 you will learn the sometimes the rotor seals get stuck in the up position and when this happens you lose compression so all you need to do is grab a spray bottle and spray some tranny fluid in the motor and the seal becomes unstuck. They use the marvel mystery oil to coat the surface's between the rotor and the housing on the cam shaft and a few other places.