As someone recently employed by a Golden Arches franchise, I will attest to using only "table salt" to salt french fries. More specifically, I'd load the fry salting device from a 4-lb paper box of commercial non-iodized table salt [1].
If you'd like to verify this fact at your local McDonalds go ahead and order some fries "no salt" and do the salting yourself. As a bonus, you'll also most likely ensure you get fresh fries. Don't do this in the drive thru, as it's kind of an asshole move. 3:20 for fresh fries.
I had lived with the lie of sugar in salt for more than a decade, glad to have that settled.
I am actually thinking of getting some of the 'Diamond' salt they use in the US. It is not just granulated, it has a 'diamond' shape to the crystals so you get the taste but not the high quantity of sodium.
If you'd like to verify this fact at your local McDonalds go ahead and order some fries "no salt" and do the salting yourself. As a bonus, you'll also most likely ensure you get fresh fries. Don't do this in the drive thru, as it's kind of an asshole move. 3:20 for fresh fries.
[1] looks just like this: http://www.webstaurantstore.com/bulk-iodized-table-salt-4-lb...