@SuperDicq I guess I was a bit ambiguous.

As engines seem to burn oil at the rate of around 1mL/km, it appears that if you go a long time between oil changes, some of the oil seems to start getting replaced with petrol, which at least offers some lubrication (but not as good as oil).

Thus about 4000km in, if you started off with 5L of oil or whatever, your lubrication would consist of mostly petrol with some oil I guess.