Dunno what it's like with the M54 thermostat but the only way to go for the M57 is OE (or as close as you can get, i.e. verified OEM supplier). They last ~10 years, where-as some aftermarket brands have been documented as lasting a year or less!

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Wayne
2005 BMW X5 3.0d (b 02/05)
2001 BMW F650GS Dakar (b 06/01)