yes. combustion gas gets past the rings, into the crankcase, and contaminates the oil. The addititves in the oil neutralize the contaminants, but eventually those additives are gone.
Also the thermal breakdown you hear about is oil boiling at different temperatures (at a microscopic level) due to the non-uniform size of oil molecules.

Just to point out- that's why synthetic is better and will last longer. Uniform molecules means a uniform boiling point, which means it resists boiling better. Which means it doesn't burn off and break down as much as regular oil. Also, being designed specifically to do one job (unlike regular oil that isn't designed...it's natural) it doesn't need waxes and things added to give it oiling properties. It's built into the base oil. That leaves room for more of the neutralizing additives, which with more additives, can neutralize more contaminants, which means you can leave the oil in longer.

Also, water is another contaminant. Leaving an engine sit can lead to moisture build-up, and that doesn't lubricate well. All those contaminants get trapped in the oil filter, which, is why it must get changed as well.

I really like the articles at www.mototuneusa.com for info on rings and blowby. Has to do with that hard break-in thing, which is used to set the rings properly. Properly set rings have less blow-by...which leads to less wear, and less oil getting into the combustion chamber.