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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?

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My best guesses:

  1. You're burning oil, and extra combustible material in the combustion chambers gives you more energy output with the same amount of gas going in (and thus better effective mileage, although you're not measuring the oil you burned).

  2. Having less oil in the engine means less parasitic loss from the oil pump and perhaps other moving parts that move through oil.

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    Check the original question: the driving habits also changed. I'd "blame" the increased mileage on the switch to mostly highway driving rather than anything to do with oil.
    – Bob Cross ♦
    Jul 22, 2013 at 12:15
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    10k miles/1 year may be OK for going between oil changes, but it is NOT a safe period of time to go without checking the oil level. An engine just starting to burn a little can burn off a measurable amount in a month of "typical" driving, and a dangerous amount in just a couple months.
    – Brian Knoblauch
    Jul 22, 2013 at 16:40
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No, the oil was most likely converted to SMOKE, not sludge. (burned, possibly) It means your engine is using oil in one way or another. It may even be leaking out from somewhere. You also need to start checking your oil level MUCH more frequently. The oil level should always be kept in the "safe" zone on the dipstick to prevent costly engine repairs.

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