I am not an EXPERT in oil, but I have had an opportunity to learn the basics of oil analytics working in a govt lab that specializes in OIL IDENTIFICATION. We used Infrared Spectrometers and Gas Chromatographs...
I used to be a skeptic of synthetic oil...figured Id just use regular old oil and change as recommended. Starting running my vehicle lube oils through the machines to see what was going on. used both conventional and synthetic.. new and used. w/o going into lengthy details I will NEVER run conventional oil in ANY vehicle I own including my RV. Conventional oil starts breaking down pretty early and by the time it got the 3000-4000 miles, a lot of the original oil constituents are gone. Synthetic on the other hand was 95-100% the same after 12 months/9000 miles or so.
I currently do a 1 year oil change with a new filter every 6 months. I keep an extra quart for each vehicle as it can still be "consumed" (blow-by, leak, etc) but at a slower rate than conventional in my experience (remember its staying in there for a year vice 3-4 months).
when I do change it, it looks a full shade darker brown, never black.
This is my experience and thoughts, it may not suit everyone. I find it works very well for me and I spend one full day a year changing oil in everything (RV, Mustang, Explorer, Dakota, VW Rail Buggy, Honda ATV, Riding mower, RV Generator, and Push Mower).
Good luck!