Preferably a fully synthetic oil if you don't want the potential for sludge, AND shorter oil change intervals than the service book calls for. I do mine no later than 10k. Always 10k or less. I have zero sludge issues. Even after 10k, the used oil is still golden, slightly darker of course.
As for Castrol, I only use the Castrol Edge 5w-40 in my E36 M3, nothing else. The Castrol is a little overkill, but if you don't mind paying extra, that's up to you. Remember that the Castrol product doesn't come in 6lt. The bastards make you buy 2 x 5lt so you can have your 6lt. And The Castrol rarely go on sale.
My Aurion gets by very well with the Nulon 5w-30 fully synthetic. They go on special occasionally at Super S**t Auto. All you need is one 6lt container and it will fill it to the top mark on the dipstick. The book says 6.1lt. I wouldn't pay the extra 20 odd bucks for the extra 100mm.
The regular price is around $67 for a 6lt. When on special they can be as cheap as $47 ! I buy 3 at least.