I understand oil analysis in aircraft where there's dire consequences for engine failure--we did it after ever "x" flying hours in the military (where "x" varied by aircraft, but could be as low as 25 flying hours).

It consistancy gives great charts. If you find a certain metal concentration changing rapidly, you either rubbed a coin between your fingers and dripped some oil on it, or you had increasing internal wear. (Note: odd but not catostrophic readings put you up for a cold sample, then another hot one after the next flight)

All readings were taken within 15-30 minutes after engine shutdown (varied by aircraft), for consistancy, with a set proceedure to get "middle of the reservoir" oil.

And you reset your graphs after oil changes (although it tracked across them)

But random sample intervals in a car engine, probably with oil changes inbetween and a wide variety of miles after oil change when taking the sample....what will it really tell you? A check 4k after oil change, then another 1k after a different oil change, would make it seem like your engine is self-healing.

If you have a gross failure, like anti-freeze, it probably gives you a little bit of a heads up. If you have a gross failure like metal wear....you don't know what the part is unless they also have metals composition for all the vehicles (you could pinpoint the part in a military engine). And if you don't know for certain, most people aren't going to tear the engine apart on a hunch.

If you don't sample often and consistantly, it's not really of much value. I'll just wait for the classic symptoms and save my $$.

If there's a better reason to do it, please let me know. I'm obviously ignorant of why to do it at random intervals on car engines.