[bluGill]

Actually long oil changes can be better for an engine. Oils break in: the molecules break down over time so they compensate by making the chains longer. As the molecules break down they get closer to the idea size, then the break again and get too short to protect.

Tests on the oils used in my car show that you typically get least engine wear between 8k and 9k miles, with 12k being about the same as fresh oil, but 13k is much worse. Because of the sudden dropoff at 13k and the fact that different driving styles affect breakdown differently 10k is chosen as the best compromise. Newer cars get longer because the computer keeps track of driving styles to give a better change indicator.

[sjwright]

That's genuinely fascinating and I'd like to learn more about it. Can you recommend some reading material for me? (Or better still an Engineering Explained-style YouTube video if someone has covered the topic well in that form.)

[bluGill]

The above was gained from reading forums. My personal judgement is the people saying the above know more about oil than me (they at least understand enough chemistry that my college chemistry doesn't see anything wrong, but I went into computer science so I didn't take more than the basic chemistry for engineers so it is possible that they are wrong on details.

One other point they brought up: every time you open the system to add or check oil dust gets in. Modern engines are generally sealed so this dust until it is filtered out (assuming it is) is doing wear as well. Long oil changes with no checking of the oil level are thus a good thing as well - so long as your oil level never gets low - low oil is far worse than any gain from not checking oil levels.