This is easy to determine - pull the fuel rail leaving all injectors attached, prime the fuel rail in position 2 - leave a colored shop rag underneath the plugs -

Come back after sitting for a few hours - the ones that leaked onto the rag (if one or multiple) are the ones that are leaking internally - they must be replaced if they are leaking down, no rebuilding will fix them.

Go with OE injectors for obvious reasons, there was a thread on E46F or BF showing that BMW updated the Siemens or Bosch injector later on in the E53 lifecycle, apparently there was a faulty design that could leak internally -

Dorin (Doru) on this forum went through this with his E39 and could chime in to help - he ended up replacing all 6. Yes its about $600, but you will never have injector problems for the rest of the vehicle's life most likely.

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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