I have an 2006 4.0L V6 (118,000KM) which was having some power and oil usage issues lately. I'm pulling a c'van around OZ so it's easy to notice changes.
Last week I had it in Sunco Nissan to sort it out and they told me that there is no problem with the engine BUT it had the wrong grade oil in it.
Nissan in MacKay had put it 10W30 at service, not 5W30. The effect of this is that the oil doesn't flow back to the sump quick enough and it doesn't lubricate the timing chain properly because it's too thick.
This leads to excessive oil usage (not burnt) and "off" timing.
I have only driven it a couple of hundred K's since then but it is a different animal now.
Compression is excellent and plugs were not fouled with oil. It did show that the mob in Perth who I paid to fit 6 new plugs only changed 3. The other 3 the manifold has to be removed.