I'm running the STP/Marathon stuff in my van. I picked-up a propaganda flyer from inside the store and it was amazingly honest. Their claim is "twice the legal minimum detergent level." They make all sorts of claims about how their gas can PREVENT build-up on valves (as compared to the legal minimum), but also clearly state that it will not clean dirtly valves, nor provide any other advantages other than those that would result in PREVENTING build-up.
In my area, Marathon is nearly always the cheapest gas. My take on the marketing is that they want to be able to say "and our gas has a little more than the minumum requiments" or "we know that its cheap, but it's not because we are skimping too much".
