Re: oil change - miles or time
The funny thing about not changing oil is that often the damage occurs in smaller increments. Unless you seriously let your oil go for very long periods of time, it's not going to immediately blow up on you. It can happen, of course, as I have seen when I worked in a machine shop back in the day. But what normally happens is that your bearings, rings, cam lobes, valve guides, valve stems, etc. etc. all wear more quickly as your oil begins to break down and becomes less viscous. This can result in a number of things happening which may or may not coincide with whether there is old oil in your engine at the time it fails because engines are wearing all the time, whether they have new or old oil in them. They just do it faster when old oil is in them.
I hate to say it, but it is very analogous to the effects that smoking has on the formation of cancer cells in the body, which allowed the cigarette companies to claim for so long that they could find no link between smoking and lung cancer. Sometimes, people can smoke 2 packs a day for 20 years and no cancer. Some smoke two packs a month and die in a year. Same is true of engines and engine oil.