On a car petrol engine it makes no difference; stick to manufactures guidelines. It wont hurt it but you are wasting your money.

On an oilburner which uses the oil as both a lubricant and a coolant or a motorbike which does 13,000-15,000rpm, then yes, every 4000 miles or so is more advisable (after all its doing twice as many cycles as your ST in every 10,000 miles).

I think Citroen service interval is even higher - 18k in my mind?!?