From previous experiences, you can't go wrong with BMW coolant and distilled water (50/50 mix). All you need to do is open the drain valve on the bottom of the radiator as well the block drain bolt (located on the pass side on the 3.0, don't know where it's located on the V8 models) and let it drain. If it's in pretty bad shape, than you will need to flush the system.

Dan

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2012 Enclave
2010 F-150 XLT
2001 330i