I have used NGK BKR6EIX ($7/each) on my:
- 1998 528i (E39) for 30K miles
- 2006 X5 3.0i (E53) for 10K miles

I will never go back to Bosch.

The NGK Iridium is simply superior. Not a single hiccup.
Smooth engine/rpm etc.

I use a small drop of antiseize and no matter what plug I use, I usually remove them at 45K or so for inspection: if still good, re-install them.

For Iridium, I replace them every 80K-90K or so.

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT