Here the story:

Chevrolet Zefira: Wife starts car, I think the sound of the engine is strange.

I check the oil --> NO OIL inside (but still the warning light stopped didn't light when the engine running so it didn't run complete dry yet)

On that cap where you refill the oil is some browny-redish residual

When I open the cap definitely air is blowing out of it. But I can not make up my mind if the air is burned gasoline or just normal air (my nose isn't very sensitive as well).

Chevrolet repair shop tells everything is OK.

But I have doubts, on my previous cars air didn't blow out of the engine. For me it sounds like: valve seal broken/cylinder had cracked/gasket broke or something similar.

Engine is a normal gasoline engine, I think 2.3 Liter (no turbo no diesel).

Am I paranoid? Oil I checked before sometimes so it seems wrong that the oil is complete finished without much km. But I might be wrong on this and forgot to check for some time.

Does it have oil in now and does it sound OK?

If it ran out of oil then that is not good but if there was enough to keep the pressure up but not enough to show on the dipstick then you were lucky.

The brown-ish residue is usually caused by water in the oil or at least condensation in the rocker cover, sometimes caused by a lot of short trips and engine doesn't get hot enough to get rid of the moisture. But unlikely in the tropics.

Check the radiator level is that OK. Coolant looks normal?

The air pulses from the oil cap could be normal, as the pistons go up and down they also displace air at the bottom so there is a natural air disturbance in the sump.

Monitor the coolant and oil levels closely they will tell you if anything is really wrong.

+1 to above post.

If no oil on the dip stick don't start the engine but top up first and then monitor closely.

Check water level in radiator (cold engine).

Is it smoking from the exhaust?

Do you mean you checked the dipstick while the engine was running?

You mean you checked the dipstick ?

Is the exhaust blowing blue smoke ?

Check around the engine and sump for leaks ? I've done on two cars, mini that had a cracked gearbox and an XR3i that had a con rod sticking through the sump.

Have you had any work done recently that may not have been put back together correctly or no new gasket ?