[tzortn]

As I recall from my college days, the oil does not break down, the additives do- viscosity improvers, anti-foamers, anti-ash agents, etc. So the OIL, still being intact, will still lube.

Any PE's here with a definitive answer?

It's my understanding that oil doesn't break down but the viscousity increasing polymers do. So, if you have 10W30 oil, eventually it will behave like 10W15 do to this breakdown of polymer chains. If that is the case, you're effectively lubing the chain with a 10W15 oil, which is too thin as far as the manufacturer's recommendation is concerned.

Oil breakdown includes the reduction in viscosity and the consumption of the modifiers. Here are some much more technical break explanations:

http://www.practicingoilanalysis.com/article_detail.asp?articleid=475&relatedbookgroup=OilAnalysis

http://www.motoroilbible.com/two.html