re: "if you check it regularly, there's no need to keep some."
M5's appear to consume oil at a rate of one quart every 1000-2000 miles, judged by postings on this bulletin board. This oil is only available from BMW dealerships, and I doubt that means small town dealerships that sell BMW's and Ford trucks. High oil consumption and sparce supply...a bad combination. When my M5 arrives, I'll always carry any extra quart with me. ...two quarts, if I'm on an extended road trip.