@ atabayat. Hopefully I have summarised your posts correctly as I try to make sense of the problem:
(a) when you bought the car (secondhand) about 4 months ago, you checked the engine oil level and it was ok.
(b) 6000 km later you checked the oil level and "it didn't have any oil on it" (presumably the dipstick).
(c) you added 1L of oil and after 1500km the oil level was showing
below Low.
(d) you have checked daily under the engine and there is no sign of any oil leakage.
(e) there is no smoke of any kind coming out of the exhaust.
(f) there is no smell at all from the engine (e.g. burning oil smell).
(g) your mechanic had a quick check, but he didn't have an answer?
Okay, I'm stumped because it just doesn't compute.
If it was my car, I'd be going all diagnostic ballistic: lift the car, remove any undertrays and thoroughly check for leaks (to be sure), then do a battery of every diagnostic test I could think e.g. scan the engine, do a compression test/vacuum gauge test/leak down test/oil pressure test (yes, tech guys, I know some of those are redundant testing (
).
I would do that because I have the time and necessary tools and a friend who knows how to use all of them (embarrassingly, I don't).
Just clutching at straws here but there are reasons for asking these questions:
1. When you added the 1L of oil, where did that register on the dipstick e.g. in the middle between the Low and High mark, or where?
2. Was the engine 'hot' or 'cold' when you took these levels?
3. Have you checked the radiator coolant? Does it smell oily?
4. If your car has a radiator overflow bottle, is the level high?
5. Have you felt any decline in engine performance since you bought the car?