2006 yz450f smoking and metal shavings -Help

So my buddy just picked up an 06 yz450f. We were riding today and all of a sudden it started blowing blue smoke out the exhaust. A pretty decent amount too. I know that means it's burning oil, but what I don't get is why it is burning oil. Supposedly it has 6 hours on a full rebuild- top and bottom. We took it back and drained the oil and took the filter out and there was some metal shavings on the filter. The oil was pretty dirty and dark gray as well, so it must not have been changed since the rebuild.

What's weird is it still runs just fine. Starts easy and idles perfectly. There is a little bit of a knocking noise, but it sounds like the decomp valve. Not 100% sure though.

Could it be the previous owner not breaking it in correctly or what? I'm confuzzled.

It's not unusual for it to run fine and blow smoke, the oil rings probably failed. 

Noises are a normal part of YZF life, but some of them are abnormal.  Oil smoke is never normal, however, and neither is gray oil.  I'd have the head off and get a look at the bore.  It could be that oil ring was installed wrong (overlapped expander or a twisted ring end) or the bore was worn to the limit and reused anyway.  Could also be valve guide seals, but the symptoms are a little different with them.

Alright thanks grayracer. I'll tear it apart sometime soon and have a look

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