Coolman

If what I assume is the compressor are the same size if this is the case at the lower condition the oil cooling load would have increased. which would mean the pipe sizing would be to small. and if your drawing is correct or as I read it. The vent from the reciever to condensor is only 50mm this is very small and also unless the liquid drain form the condenser is massively oversized, meaning the actual thermosyphon reurn line is limited to the 50mm vent line.
Why dont you let us know the compressor type and speed and conditions and we can check thermo line size etc.
regards
RWF II