I drive a 23i.
Just ignore the oil until the light comes on which says you need to add 1 litre of oil. I usually carry 1 litre in the boot just incase.
I bought 4L of Castrol EDGE 0W-30 FST from Opie Oils for £38 and use that to keep my 1 litre bottle topped up in the car.
Pouring in a litre of oil takes less than 60 seconds. I usually just pull over into a petrol station to do it when the light comes on.