It doesn't take much to keep the inside of an engine clean - here is a Toyota engine from the late 90's with ~270k miles on it (my 4Runner):
That's 5k oil changes with whatever the quick-lube place put in it for the first ~250k miles and then a couple oil changes with Rotella T6.
Having worked at a Toyota dealer for a summer around 2000, I can tell you that there are TONS of Toyota owners that are horrible with maintenance. About once a week, a customer would come in with a seized motor around 60k miles - because they had literally done zero maintenance (no oil changes, no adding oil, nothing)... My wife's first car ('95 Tercel, bought new for her at 16) made it to ~150k miles with 3 or 4 oil changes (and yes, it died of spun bearings). It had more sets of tires than oil changes. That's how those 'sludge' issues happen...
-Charlie