I just replaced my plugs this weekend, 66K mls, dead easy to do. It now idles smoother from a cold start. I went with the revised NGK plug that produces a more powerful spark, which might be reason enough to change them early, but I've also read that as the plug becomes weaker towards the end of it's life, it stresses the coil more, and may lead to it failing sooner. I don't know if that's true, but given the cost of coils, seems like cheap insurance.
In the photos of the plugs above, bear in mind that the plugs shown aren't the same
. NGK revised the plug for the N52 for 3/2007, so the difference you see is probably that revision. The old plug is NGK part ILZFR6D11, and the new is PLZFR6A11S (I can just make out the part number stamped in the collar of the new plug in the photo above.) Make sure when you buy new plugs that you're getting the updated plug, I saw both avail for purchase on-line, including at the partsgeek link below. Realoem parts page:
I bought mine from Partsgeek for under $11 each.
The gap for the PLZFR6A11S is 0.43. The Bentley has the torque as: 23nm +-3 nm (18 +- 2 ft-lbs) with copper-based anti-seize, hand thread initially to prevent cross-threading.
Also, since you'll have the engine cover off, clean under the rubber gasket around the oil fill cap. It's designed to trap any oil that's spilled when filling.
Update: after the change, it would rough idle for short periods of time. I read that with a BT tool I could clear historical info in the ECU so it relearns with the new plugs, the alternative is drive it enough miles so the previous plug info is aged out, and that works. Took ~500 miles. And yes, the new plug did make a difference, it's stronger in the mid-range.