I don't know either. Just keep in mind that ExxonMobil is one of the two biggest refiners of Group-III base stocks in the world, and sells the technology to do so, from plans to a complete turn-key operation, to other refiners who desire it. My personal Kentucky Windage guess is that the Mobil Drive Clean Synthetic Blend is composed of Group-II and Group-III base stocks and an appropriate additive package. If you want to assure you're getting the good stuff at the best price, use all but one quart in your sump of Mobil Drive Clean in whatever viscosity grade, and then finish with one quart of Mobil 1. In a 5 quart sump that's nearly 20% Group-IV and Group-Vs by volume - a tiny percentage of that being the additives. Rarely do the preformulated synthetic blends contain more than 10% "synthetic" (whatever base stock is used as the "synthetic") by volume. Rolling your own with same brand oils would probably be easier on your wallet, too, as well as minimize the overblown "risk" of additive clash.
While I have had excellent results with M1 the Drive Clean Blend has not performed for me ever. When I put the Drive Clean Blend in my my vechiles will burn it up like I just put Wesson in but with M1 I have never burnt a drop. It makes my think that it is group I with some group III added. I can not prove it just a guess! If drive clean or drive clean blend works in your engine then go for it.