Fries aren't cooked in the same oil as the fish fillets and nuggets because of vegetarian concerns. Never tried the fries in any oils other than its oil (mainly because they were in different areas in the building).
The oil of the fried chickens (the nuggets, McChicken patties and Crispy Chicken patties (maybe more or less different types depending on what country you are from)) are kinda interchangeable.