Oil change places and oil companies have a vested interest in promoting the myth of frequent oil changes. Recent studies show 10,000 intervals as more than adequate for most new vehicles using full synthetic oil (the only kind anyone should use). I've put a quarter million miles on cars that sometimes went over 20,000 between changes and never had a problem. None ever even used a drop of oil. Shops tend to foster all kinds of myths to earn money. Like brake shops who tell you you need new pads at 20,000 miles (a physical impossibility. Modern brake pads can go as far as 200,000 miles).
Maybe their fleet arrangement is dispose at 60k miles. We have 4 year new vehicle tags in our state, and at 60k it's done anyway. The emission tests for the smokers or oil burners is not an issue. Would you want to be the next owner? Our company does every 5000 miles.
Beyond the vehicle cost is the business revenue lost with an out of service vehicle. The only other reason a company may elect to do 12k miles is simply being cheap multiplied by x amount of vehicles to some. Others want to keep their vehicles up and presentable. We also keep our cars washed and cleaned.
Part of what you said may be true if you are doing every 3000 miles. But dusty roads, short stop start drives that never fully warm the engine, extreme temperature areas like Az, I would change often. Oils can breakdown with heat.
I don't run synthetic in my old cars. I change my synthetic every year on my Corvettes, even at 1000 miles driven. You can never make a mistake changing your oils more often, and stay away from cheapo generic oil filters. And the same goes with air filters. No mention has been discussed on clean filters. You suck in dirt right to your engine. I have dual filters on mine, and even blow off my drive with pine needles, leaves, etc that get sucked up on my low profile cars. Every mile driven on the open road, there is dust that may or may not be stopped by your air filter. Doing that 20000 miles, even with synthetic, seems like bad advice.
Changing oil often is never wrong. But with oil changes costing over $100 now, it can get expensive. When full synthetic oils were first brought onto the market, the recommended change intervals for them were 25,000 miles. As soon as oil companies realized such long intervals would cut into their profits, they quickly back-pedaled and start advocating 10,000 mile change intervals.
Full synthetic oils do not degrade or break down. I've torn down engines run exclusively on full synthetic, and found everything inside to be absolutely clean and polished looking, with only a thin film of oil on them. With cheap, non-synthetic oils, there is always a good quarter-inch of sludge, and everything is black and gritty.
The variable in your theory is the air intake. Dust enters the engine and starts the wear.
The other issue is start ups that never fully warm up the engine and allow water to build up. A short trip to a store, or move a car out of a garage and park outside. Never warmed up and condensation builds.
The best investment you can buy if you like cars is an electric home car lift kit. Easy to put together and no foundation work. Costs under 2 grand. I had a contractor raise the garage attic for 800. You gain a third parking space in your garage. And increase your home value. Park a car up top and one below I roll my drain pan in place and open the drain plug, and go in and watch TV. Then do the filter. Doesn't get any better, and my cars last.
I've religiously changed my oil every 3,000 miles. The other thing, is that I've seriously only ever used whatever oil happens to be on sale - so basically the cheapest stuff. As long as it states on the back that it "Meets or exceeds manufacturer's warranty" then I'm fine with it. Almost all oil made today has some sort of detergent in it anyway that keeps the 'innards' fairly clean. I do this because it's cheap insurance.
That said... my brother owned an old Avalon up until a year ago and he seldom changed the oil. As whenever it did get changed, what came out resembled coal tar. I was amazed it even ran, yet somehow it made it to around 310,000 before he sold it. Yes - that was luck of the draw, but why chance it?
People will skimp on a small car that takes only 4 quarts of oil and filter. And then not bat an eye for a 4 grand new kitchen backsplash for their home as an example.
My family does a lot of rebuilds. For something that is used everyday and is expected to do many things, it is amazing how nonchalant people can be about proper maintenance. Then many have to wait til Friday to have the money to pick them up. Or they are babysat for a week til they can pay for repairs. Not all have credit cards. It's a good study about human nature and priorities, seeing this in real life. Changing oil and filters is really a no brainer. A classic case of pay it now or pay more later. Many drive their vehicles into the ground. But overspend on a lesser priority. Pretty amazing to hear.
I wonder if the person who bought your brother's car knew it was run for so long with "tar" as oil. Over the years, the few times I have sold one of my high mileage cars or trucks to someone wanting to keep it on the road, I've always fully disclosed any mechanical problems. I've even told them about anything that might soon need attention, like the brakes, if I hadn't replaced them for years.
When I decided the vehicle needed too much work, and was unsafe or not roadworthy, it just got scrapped.
You can't count on everyone selling a used car to have honor above reproach. I have signed many Bill of Sales as a buyer that say "As is where is". That means buyer beware and it's your call with zero recourse later. It's good to look it over as best you can, and/or get it on a lift, and so on. I am not an expert on all cars, but have learned a lot. When I go into a garage and see the private owner has 6 new high quality filters on the shelf, it's usually a good sign. Sorry to say that you can't trust everyone or why they are selling.
In the case of my brother's car, it had over 310,000 miles on it at the time of sale and the buyer was well-aware of this. When we changed the timing belt and removed the valve covers, we were surprised to find that the valve train was spotlessly clean. Before the sale, I did a compression check and it was still within acceptable pressure. So the car was fine, but again, we're talking about an elderly car with a LOT of miles, and so anyone buying a car like that is bound to be taking some risk given the excessive usage.