I think Merc specify "premium plus" as an alternative to Opti if this is not available ...... not "std" premium.
Given the low useage and the price difference between the two, I have opted for the DFI oil ........... expensive engine and the gold stuff smells nicely disgusting
A commercial operator I spoke to in South Wales had been running premium plus for two years with no problems, but its mostly fast/ WOT trips around the bay ........... with a bit more trolling as I do I "believe" that the DFI oil helps prevent plug fouling ??
Whatever .... its the buyers choice I guess