I worked with Lewis when they manufactured IQF freezers.. Almost all coils were top feed so they wouldn't pocket oil. As aphun mentioned, he had to install an oil purge valve...but you say this has been checked.

However, you also state no oil pot on the pump vessel... Is the pump vessel free of oil??

If suction line is short (minimum temp. gain), suction lines not blocked (valves open and no strainer blockage) pumped liquid temperature should be very close to compressor suction temperature....If not, oil or water contamination are suspect..