I think the truly careful grammatical answer is that you don’t form a plural of a possessive. Instead you form the plural of the object being possessed – in this case, McDonald’s restaurants. Similarly with book titles – it you have a stack of books, all titled “The Grapes of Wrath” you don’t have a bunch of Grapes’s of Wrath or Grapes of Wrath’s. You have a number of copies of The Grapes of Wrath.
Of course, we aren’t always careful, especially in spoken language, in which case the question isn’t how to spell it but how to say it – how many zs do you say – McDonaldz or McDonaldzez. But if you’re writing something you care enough about to get a rather arcane point of spelling correct, you might as well go all the way and complete the possesive.
“non sunt multiplicanda entia praeter necessitatem”