In a 2008 Jetta, 2.5L petrol engine, I started driving more on the highways, and taking longer trips, and noticed a much better mileage than the prior driving that was still mostly highway, but involved some city stops.
I then checked the oil level, and it was non-existent. I added two containers of oil, about ~2L (IIRC, engine capacity is ~6L total), and then the level was at the lowest allowable.
At that time, there was probably 10k miles after the prior oil change of about a year prior, which was done at the dealership, with obviously supposedly "fully" synthetic oil.
Could some of the lost oil, which was supposed to have been dealer-grade "full" synthetic (California), been converted into sludge?
Why did the car had great gas mileage when being so low on oil?