I use Castrol SuperClean. Really powerful stuff to get the heavy grease/oil off the block. I've done this a bunch of times on the X. I put an old towel around the alternator then put a plastic shopping bag over it. That's it. I either use my garden hose or a steam wand at the local DIY car wash. Soaking with degreaser for a while (1hr) will help big time.

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2006 X5 3.0L - Schwarz Black/Truffle Brown
Stock
But I keep it clean