My thought would be if you're doing regular oil/filter changes, you're already cleaning out the sludge and soot when you change the oil. Now I've not seen the inside of engines that have run 10,000 or 15,000 miles between oil changes so I can't say what the inside of those engines looks like. Maybe they could use a clean-out product like this, but I'd need to see what it looks like before I throw snake oil at it.
I've always heard that doing a transmission flush on a high mileage automatic transmission that hasn't seen regular maintenance can cause real issues due to the sludge dislodged. Never have had it happen to me, but it seems logical. Don't see why the same thing couldn't happen to an engine. Granted it doesn't have the valve body and such of a transmission, but there are small galleys and such.
