Granted, it costs less to make an inferior oil. That makes total sense. But I'm using quality oils and filters, or are they quality oils in the first place. The typical oil I'm using is the 20k oil, which I dump at 3k. So, why in the world would their be sludge buildup in the engine at 70k ? I went into the shop bay, and seen it myself. I'm not using additives, like say STP blue bottle, or anything like that. Then I'd say, " Ok, it's the blue bottle causing this mess" Not beating the car to death, not running it hot. It's just a daily Joe Schmo driver. It gets it's highway workouts. Regardless, it has 614, 877k on it now. I attribute that to the 3k oil changes. 13' K24 Honda engine. I can't really complain. Same bearings/cam bearings/rod bearings/cap bearings. I just think this 10k , 20k, whatever k oil is a bunch of bologna. Frequent OCI's is the key to longevity. Big Auto extended OCI intervals to blow an engine before it's time so you either rebuild it, or buy a new car.