This one looks trivially false to me… McDonalds doesn’t even use mayonaise, do they? Now, if the claim were semen in the Big Mac special sauce, or in the tartar sauce used on the Fillet O Fish, then it might be plausible. But what McDonalds item uses mayo?
I think that the ‘special sauce’ is quite commonly referred to as mayonnaise here, but they are always changing the menu here anyway (to include novelty stuff like ‘Greek Mac’ etc, so I suppose real Mayo is going to feature sooner or later.