FWIW, I used these parts on my install:

BMW E53 X5 M54 3.0L Engine Drive Belts ES#517967 Hydraulic Alternator Belt Tensioner Conversion Kit - 11287838797

BMW E53 X5 M54 3.0L Engine Drive Belts ES#259467 Idler Pulley - 11287841228

BMW E53 X5 M54 3.0L Engine Drive Belts ES#14608 Idler Pulley Securing Bolt - 07119904677

BMW E53 X5 M54 3.0L Engine Drive Belts ES#21662 Protection Cap - 11281730349

BMW E53 X5 M54 3.0L Engine Drive Belts ES#205356 Mechanical A/C Belt Tensioner Assembly - 11287512758

http://www.ecstuning.com/BMW-E53-X5-...Belts/ES10333/

http://www.ecstuning.com/BMW-E53-X5-...Belts/ES10334/

The Genuine BMW belts are made by continental so I didn't have a problem using the Contitech belts in place of it, to save a few bucks. The whole tensioners/belts/pulleys replacement was done over a year ago on my X and not a single issue. For what its worth, I already had a hydraulic tensioner kit installed on my X but I bought the "conversion kit" from ECS because I like how they package new hardware with it.

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2018 Ram 2500 6.7L Cummins 68RFE
19k miles -Bright White/Black - Big Horn Sport - Crew Cab Short Bed
2013 X5 35D (CEO's) - Born on 5/17/2013 -
82k miles - Alpine White/Cinnamon Brown/Premium Pkg, Sport Activity/Premium Pkg and Sound/20" Style 214/Running Boards