I've run BMW 5w30 (Castrol TXT), Castrol 0w30, Lubro Moly 5w30, Lubro Moly 5w40, and Mobil 1 0w40 in the M54 engines. Never noticed a difference between any of them in how they run...personally using Castrol 0w30 in our `02 325iT.
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Update: I've gone nearly 1000 miles on the clean phase while running Rotella T 15W-40 and ARX. The car has developed a hard starting problem after sitting for a few hours. I think this may be due to the thikness of the oil interfereing with the VANOS system. It has me worried about the well being of my engine and I am considering stopping treatment and resuming with a 30 weight oil.
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I'd bet my 0W30 is still thicker at 30C than your 15W is in the current weather conditions.