I'd absolutely advise you to use synthetic oil on your BMW and replace it every 5K miles. I don't like Castrol - that's also the BMW OE oil. I'd suggest to use Mobil1 5W40 in FL. Regular mineral oil won't resist heat and pressure as much as a good synthetic oil. The problem is with BMW extended 15K miles oil change interval, which is a sure way to ruin your engine even with the best oil. Use good synthetic and replace it at every 5K, it will keep internal seals (in VANOS) and all oil seals in better condition also while preventing sludge built up.