A PA31 Captain reported returning to departure airport after shutting down the left engine for low oil pressure because his trainee pilot failed to replace and secure the oil dipstick.

Narrative

During the pre-flight; my trainee added a quart of oil to both the left and right engines. I monitored his adding oil to the left engine; however I did not watch him add the quart to the right engine because I had to run inside to complete some pre-flight paperwork. When I came back out to the aircraft; our cargo was waiting to be loaded and in my haste; I failed to verify that the right engine was secure for flight. After loading up the cargo; we proceeded to the active runway and began our flight. During the climb to 7;000' we monitored our engine gauges as per procedure and everything was within normal operating limits. Upon reaching our cruise altitude of 7;000' MSL; we began to lean out our mixtures as normal. When we attempted to lean the right engine; we noticed that the engine was unable to maintain 26' manifold pressure. I then checked the right engine gauges and noticed the oil pressure around 10-20 PSI (40-30 PSI below normal) and dropping. I then glanced out to the right engine and saw that the oil door was open and leaking oil. I asked my trainee if he had replaced the oil dipstick and he replied; 'I thought I did.' Obviously; he did not and we were now losing oil. Immediately; I made the decision to shutdown and secure the right engine. I contacted Approach and reported the shutdown and told them that I wanted to return. They immediately vectored me back and told me to report the airport in- sight. I did and they cleared me for the visual approach. I followed the company's Emergency Procedures and safely secured the right engine and landed safely. To prevent this from happening again; extra vigilance must be taken to insure the aircraft is ready for flight. As pilot in command; it is my responsibility to complete one last walk-around to verify that all the gas caps; cargo doors; and oil caps are secure. Had I taken the extra minute to do this; this entire situation would have been avoided.