The biggest difference is that WD40 and other penetrates wind up washing off with regular driving in the rain and do so within a couple of months if not sooner. Dirt also sticks to those other types of protectors much more than say MPHD. They also don't protect very well against salt intrusion in areas where salt is a factor, like northern climates with a lot of road salt in winter and beach areas. The other metal protectors mentioned stay on the vehicle and protect much longer. That being said, I know some folks that actually coat their under carriage with used motor oil. They swear by it, but I have never seen before and after pics or any evidence that it doesn't attract dirt, etc.
