Black Hole. Although Ultra Glaze is the better filler, IMO.
Wax will bond better to cleaner paint, so if the Merc is less contaminated then it will last for longer.
The merc is a family ( battered) car and is in no way less contaminated than my freshly sprayed ( bonnet, bumper, wings) but does seem to hold the wax for longer , very odd !?!
Thanks for the links