Don Weed, who is a Mercury Master Mechanic and moderator of the BBC Mercury forum, advises that Mercury now recommends Optimax/DFI oil for all ProXS models due to it having more additives to fight carbon build up than the Premium Plus. My dealer confirms this. It is also fine to mix the two while you're changing over. I have done the change over on my 250 ProXS.
AFTER you mix the two oils give your mechanic a call. the oils will gel in the tank if mixed together. It has caused 2 powerhead failures in my area already.