Well, I have a slighly differing viewpoint.
I don't want the old oil in my engine much longer than 3-4K. Is it still lubricating? Yeah, probably. Is it still suspending soot and carbon particles and dumping them into the filter? I hope so. Is it still neutralizing acids? Hell, I don't know but I hope so.
Now, I know all of you want to say, "get the oil analysed and find out", but what if I don't want to do that? The cost of Terry's analysis is more than a Mobil One/Pure One oil change. Any small cost savings incurred by extending change intervals has been wasted on the price of an analysis; I haven't done the math but off the top I'd say the cost uses whatever savings from extended intervals.
The money simply doesn't matter to me. I'd care if an oil change costs, like, $100 but I can do it under $25. I've never paid more than $4.00/qt for Mobil One. I buy my filters when they go on sale for well under $5.00 for a Pure One. To me this is cheap insurance and probably the best cost/benefit maintenance an owner can do for their car. So, yeah, I'm yankin' out the M1 at 3-4K and no more. No big deal to me. I'd much rather have the old stuff out (regardless of how well its still performing) than try to extend intervals in order to obtain some benefit that is not either tangible/measurable or has no value to me.
My #1 goal is to have a perfectly running car with an engine that is receiving the best lubrication possible. If it costs me a couple bucks more per year, that's money more than well spent in my book.