I hate to follow myself up, but this one other thing occurred to me and I wanted to present it to you all for your consideration too.
When you've got contaminated oil there are actually two components to the contamination's effects. The contaminant will, to some level or another, exhibit both, but one will be prevalent and we often forget the other to our detriment. A contaminant may be either erosive or corrosive. Erosive is easy to understand, you get dirt in the engine and it acts as an abrasive and wears away metal - no problem grasping that one. But there may very well be a corrosive element to it as well, and these are the chemical reactions that form acids or cause internal electrolysis that eats away at parts too. This second part gets too little attention, but its really one of the main reasons for changing oil. The stuff you dump out is not the same stuff you put in + some worn metal; chemically its a much different soup.
Oil changes are cheap, always cheaper than metal.
