Sandro,
Yes I am refering to oil seperator level but this one is a bit unusual.

I havn't had agood look as to what is happening, but in one instance I drained
50 litres of oil out to see level in top sight glass in oil seperator.

When it started oil level was to low?

I didn't personally see it but somethings a miss.
The only place for oil to drain back when compressor is off, is oil filter housing in compressor
as far as I can see!