Then you must have mysteriously had an unusual steady stream of bad "Imports". I'd be curious what they were. Yugos? I'm curious you see because I've owned a number of "Imports" - specifically Toyotas, and none of them have had any major issues, and we generally keep them for 250-300k before trading.
So what does that mean? Just because you have 200k plus mileage, doesn't mean you don't have oil leaks or burn oil. I have had a couple that needed oil every 500 miles and were high mileage. I bought a car where a guy was using 20w50 thick oil as he felt it was better. Not a great oil for winter cold start ups. People pour in STP, Marvel Mystery Oil, Seafoam etc, etc, which I do not like dumping in my cars. The old timers used kerosene to clean the sludge. I do not use or burn oil, or have leaks.
Maybe you don't have a problem, but your broad advice might not be true for the next guy. You might be harming your new Maserati vs the Yugo. No one stated what car it may cause harm to. You shouldn't be needing all these liquids if your engine is well maintained. I don't even use the hot pressure washers under the hoods to affect the seals. Or bend my radiator cooling fins. And don't hose under the hood to short a modern computer driven car. My parents have seen this with their shop. Cars are expensive and you can do great harm.
The other thing I have seen is people destroying their paint finishes and high dollar custom wheels with acidic wheel cleaners. Just takes minutes. It's a shame as much can be prevented.
Persons with many years of experience keeping their engines healthy and running right would have little if any chance of excessive carbon build-up. I won't be adding Seafoam to my oil when my car is in winter storage, and I doubt owners of expensive high-performance V-8's like the G.M. LS series would risk damaging a $10,000 motor.
Seafoam is effective for its intended purpose of cleaning carbon and sludge out of old motors. Even in this case, there can be problems. I looked up reviews on this product, and they're not all positive.
I didn't say all people should put Seafoam in their cars, but that I've had good luck with the stuff. No matter what car you own, chances are good that the throttle body will need occasional cleaning, which is more or less routine maintenance. I've used Seafoam for that purpose and it works a lot better than some of the other things I've used, like carb cleaner, which in my opinion is way more caustic.
I've worked on all types of cars for forty years. I always advise every car owner to add injector cleaner at every oil change, use belt dressing every six months or so, add seal restorer to the oil as a preventative maintenance after about 80,000 miles, and to use only full synthetic oil (which can go 10,000 to 15,000 miles between changes easily with no harm).
I had one friend who only changed oil three times in 100,000 miles in his Cadillac (before trading it for a new one).
One of my personal cars has had only four oil changes since 2007.
People waste a lot of money on unnecessary maintenance while totally neglecting other areas of maintenance. Even some luxury car makers now only recommend changing oil at 10,000 to 12,000 mile intervals. Many have gone to computerized oil monitoring systems that let you know when the oil needs changing. In those vehicles, it is not unusual for the "change oil" alert to appear at well over 10,000 miles. My best friend's new Focus did not show a change oil alert until after 13,000 miles.
Our family has a shop as well. Many of the vehicles are high performance. However they have seen just about every type of car, from commuters to limousine service. And even inboard boats. They never use spray belt dressings. All my personal vehicles now run ridged Gatorback belts that do not slip. In fact they want to know if belt dressings were ever applied, so as to clean it off thoroughly before installing. Keep vehicles well tuned with quality fluid changes and good filters. You do not need all the chemicals unless there is an issue. Engines and transmissions are expensive, and they have seen it all.
I am hesitant to switch to synthetic myself on my big block. I use Castrol and ZDPP due to the loss of lead in fuels, and change twice a year in spite of low mileage so as to avoid sludging. My other cars are running Mobil 1 however. If you bring a vehicle in, it's good to inform them what you have done. They also like to hear if you put slime in the tires when they do a repair, vs telling them afterwards.
We had one guy that said don't mind the blue smoke, she's a racer! Most issues are neglect and stupidity, there's no other way to put it. The ethanol in fuel presents some challenges, especially during storage. That's when products like Startron are good. Doing too many fuel cleaners is not good however during routine driving.
Let's ponder why a manufacturer would want you to extend oil changes. If they wear out quicker, who benefits on a replacement new vehicle purchase? Oil and filters is dirt cheap for the benefit gained. If it's dusty where you live, maybe twice isn't enough. I bet the oil filter on yours is on complete bypass. I don't feel like sanding the internals on my block. Or dropping the oil pan and using a scraper on the sludge in the bottom of the pan. How about a warranty issue? Tell Toyota you changed the oil at 13000 miles, and update us on the free warranty repair.
Yeah, and Toyota recommends to change the oil every 7,000 miles, but even if you change it every 3,000, the engine is still prone to sludge.
The oil life monitoring system in my G.M. vehicles has always indicated change engine oil before 6,000 miles. I don't see why oil would last more than twice as long in another make.
Why even stretch out an oil change? It's the blood of the engine. You are still circulating dirt and contaminants as the filter loses efficiency. Then the moisture and water buildup in the oil from the weather changes. Personally I have little sympathy for neglect. It's so cheap to have done or do yourself vs the cost of the engine. I have seen people with oil levels nearly 2 quarts low, and coolant reservoir, power steering and brake fluid very low as well. That's the bare minimum that anyone can check. Some cars you can't check the automatic trans fluid as there isn't a stick. And many don't have a radiator cap so you refill the reservoir with the "correct" anti freeze. At least with Audi you can check the trans from the dash.
I don't know about the rest of the world, but in the UK extended oil change intervals came about largely because of demands from fleet managers for servicing every 12,000 miles.
I doubt if there was any technical justification for it. In fact I remember reading about the service intervals on a Renault, which were 12,000 miles for a fleet vehicle and 6,000 for a privately owned example (despite them being mechanically identical).
Most oil "consumption" problems are actually down to owners not checking the level correctly (i.e. not allowing the level to settle for several hours).