I double checked W.Stoecker book, IIAR Piping Handbook, ASHRAE Handbook 2006, Frick manual and I didn't find any suggestion that return line can be connected to the discharge line. However everybody suggest that liquid goes to the oil cooler, part of this liquid will evaporate and mixture of the liquid and vapor will go back to thermosiphon receiver. Movement in this circuit will happen because liquid in supply line are more dense that liquid and vapor in return line. This is the reason we call it thermosyphon. This is overfeed system has typical rate 1:3.
One remark from W.Stoker book p.387. '"Separation of liquid and vapor indeed also takes place in thermosyphon receiver, but requirements are not stringent. Some liquid mist could pass through the vent line to the condenser inlet without an adverse effect, but it should be emphasized that return of the liquid and vapor from the oil cooler should not be delivered to the consener inlet, because such a large fraction of liquid would degrade the condenser performance".