Good day all. I would like your opinion. If deserves moving to engineering, thank you.

This vertical shaft air compressor oil pan has a level sight, and a fill plug. Shows full level of the new and proper lubricant oil. Starting the compressor, the sight shows no oil presence. No oil shown at the sight while running.
Stopping the compressor, the oil takes like 30 minutes! to show at the sight. But if when stops, the fill plug is removed; oil immediately fills the pan and shows at the sight.

Asking 'experts', say "never noticed/check the sight when running" and... "if shows full oil when stopped, run it and forget it"

But how to know if the oil pump suction tube is below oil surface and absorbing oil at the bottom of the pan ? Paranoia is killing me.

I suspect the crankshaft upper chamber is creating some vacuum; or the pan is creating some pressure that with the help of the dividing mesh, oil does not flow down freely. There is no venting devices nor any debris.

There is 2 'chambers' : the crankshaft above and the oil pan below, divided by a fine mesh shown below. Oil is pumped centrally up to the crankshaft chamber and does not flow back to the pan trough the fine mesh in the second image. Stays on top of the mesh while running 😳.

What is going on ? I do not want to damage it. Thank you.

I would suspect some seals are compromised, resulting in pressure\vacuum moving oil into places it shouldn't go, and when the fill plug is removed to equalize pressures, the oil quickly returns to the oil pan.

You may need to disassemble to find the problem.

1 hour ago, Externet said:

Good day all. I would like your opinion. If deserves moving to engineering, thank you.

This vertical shaft air compressor oil pan has a level sight, and a fill plug. Shows full level of the new and proper lubricant oil. Starting the compressor, the sight shows no oil presence. No oil shown at the sight while running.
Stopping the compressor, the oil takes like 30 minutes! to show at the sight. But if when stops, the fill plug is removed; oil immediately fills the pan and shows at the sight.

Asking 'experts', say "never noticed/check the sight when running" and... "if shows full oil when stopped, run it and forget it"

But how to know if the oil pump suction tube is below oil surface and absorbing oil at the bottom of the pan ? Paranoia is killing me.

I suspect the crankshaft upper chamber is creating some vacuum; or the pan is creating some pressure that with the help of the dividing mesh, oil does not flow down freely. There is no venting devices nor any debris.

There is 2 'chambers' : the crankshaft above and the oil pan below, divided by a fine mesh shown below. Oil is pumped centrally up to the crankshaft chamber and does not flow back to the pan trough the fine mesh in the second image. Stays on top of the mesh while running 😳.

What is going on ? I do not want to damage it. Thank you.

Surely you know the oil suction tube must be successfully picking up the oil, because if it were not, the oil level would not go down when the machine is running. As for why the oil immediately runs down and fills the pan when the filler cap is removed, that suggests maybe that there is some +ve pressure in the system that is released when the filler cap is removed and this pressure for some reason inhibits the oil from returning to the sump. Is there an escape of air when you remove it? But if that is happening it is hard to know whether is by design or indicative of a fault without knowing more about the compressor design. The oil is clearly going somewhere inside the machine and eventually returns when it stops, so it is not being lost. That suggests to me it may be normal. The handbook ought to say something about the oil level, when to measure it and so on, which could give a clue.

The sort of guy who might know is @sethoflagos , who seems to have oil industry engineering experience.

Oil sitting in the oil pan isn’t doing the job it’s meant to do - that’s just the reservoir. When it’s running, the oil is actually lubricating parts.

5 hours ago, Externet said:

Good day all. I would like your opinion. If deserves moving to engineering, thank you.

This vertical shaft air compressor oil pan has a level sight, and a fill plug. Shows full level of the new and proper lubricant oil. Starting the compressor, the sight shows no oil presence. No oil shown at the sight while running.
Stopping the compressor, the oil takes like 30 minutes! to show at the sight. But if when stops, the fill plug is removed; oil immediately fills the pan and shows at the sight.

Asking 'experts', say "never noticed/check the sight when running" and... "if shows full oil when stopped, run it and forget it"

But how to know if the oil pump suction tube is below oil surface and absorbing oil at the bottom of the pan ? Paranoia is killing me.

I suspect the crankshaft upper chamber is creating some vacuum; or the pan is creating some pressure that with the help of the dividing mesh, oil does not flow down freely. There is no venting devices nor any debris.

There is 2 'chambers' : the crankshaft above and the oil pan below, divided by a fine mesh shown below. Oil is pumped centrally up to the crankshaft chamber and does not flow back to the pan trough the fine mesh in the second image. Stays on top of the mesh while running 😳.

What is going on ? I do not want to damage it. Thank you.

I was a film projectionist responsible for a pair of 35mm projectors for many years. One of the critical lubrication points of a projection mechanism is the crossbox which houses the maltese-cross intermittent gear that provides the stop-start motion that draws film through the projection gate. You never start a projector up without first checking the oil level in a transparent glass on the side of the crossbox just below the filler plug - the oil level should always be halfway up the glass.

But this visual check can only be done when the projection mechanism is at rest. The moment you start the motor, the oil level becomes chaotic and unreadable. When you stop the projector, you have to wait several minutes for the oil to drain back into the reservoir before you can obtain any useful reading of the level in the crossbox. This sounds rather similar to the effect you describe. Basically it’s quite normal  - (in projectors at least).

3 hours ago, swansont said:

Oil sitting in the oil pan isn’t doing the job it’s meant to do - that’s just the reservoir. When it’s running, the oil is actually lubricating parts.

Any motor that relies on oil for lubrication has to have an excess of oil to prevent oil starvation and loss of the film preventing metal to metal contact.
A typical automotive engine that takes 5 l of oil per refill, will usually have 1 l circulating through the engine, while the other 4 l are in a baffled pan to prevent starvation of the pick-up when maneuvering.
Now granted, an air compressor does little maneuvering, but it still has to show some level in the pan, while running, to prevent starvation.
If there is none, but returns after shut-down, or on opening a 'bleed', it must be going somewhere.

Maybe @Externet can put on a pair of safety glasses, and open the bleed while the compressor is running; if there is pressure ( or vacuum ), it may indicate piston ring blow-by, and the pressure will displace the oil. This is typical for an automotive engine, but a compressor might be more likely to pull a vacuum on the pan.

11 hours ago, exchemist said:

The sort of guy who might know is @sethoflagos , who seems to have oil industry engineering experience.

I don't really see any issue here. You and @swansont have pointed out that the sump oil level must obviously fall during operation if the lubrication system is to do its job. Part and parcel of this is that hot, pressurised air must migrate from the crankcase and valve gear into the sump by displacement.

When the compressor is switched off, the cylinder fins help cool and depressurise the upper air spaces more quickly than the sump creating a backpressure that partially inhibits oil drainage return through narrow ducts due to surface tension effects. Unless someone unscrews the oil filler plug allowing pressures to equalise.

3 hours ago, sethoflagos said:

I don't really see any issue here. You and @swansont have pointed out that the sump oil level must obviously fall during operation if the lubrication system is to do its job. Part and parcel of this is that hot, pressurised air must migrate from the crankcase and valve gear into the sump by displacement.

When the compressor is switched off, the cylinder fins help cool and depressurise the upper air spaces more quickly than the sump creating a backpressure that partially inhibits oil drainage return through narrow ducts due to surface tension effects. Unless someone unscrews the oil filler plug allowing pressures to equalise.

Yes that sounds plausible.

@Externet seems to be worried about oil starvation if the sump level drops enough to lose suction. However if the resting oil level is as it should be, it seems unlikely that this is an issue. I have no feel for the volume of oil in the galleries of a machine like this, but I note the sump pan is quite small compared to the size of the 3 cylinders and the substantial-looking crankcase.

I suppose the +ve crankcase (and sump) pressure may be a consequence of a small amount of blow-by past the piston rings.

5 minutes ago, exchemist said:

Yes that sounds plausible.

@Externet seems to be worried about oil starvation if the sump level drops enough to lose suction. However if the resting oil level is as it should be, it seems unlikely that this is an issue. I have no feel for the volume of oil in the galleries of a machine like this, but I note the sump pan is quite small compared to the size of the 3 cylinders and the substantial-looking crankcase.

I suppose the +ve crankcase (and sump) pressure may be a consequence of a small amount of blow-by past the piston rings.

It's a closed, fixed volume system, so it pressurises during operation by Gay-Lussac's Law. This both reinforces the seals and helps avoid pump cavitation.

Good engineering and use of the specified viscosity oil should prevent suction starvation. I'd be a little concerned about dumping pressure rapidly while the engine is hot and then resealing as this will pull a vacuum for which the oil seals may not be designed to withstand.

If it works, don't fix it.