Cessna 172 flight instructor reported oil appearing on the windscreen during cruise. Flight returned to departure airport.

Narrative

Shortly after reaching cruising altitude; we noticed a small amount of oil appeared on the windscreen which was coming from the oil door. I immediately took control of the aircraft; started a turn and notified ZZZ Departure that we wanted to return to ZZZ due to some oil on the windscreen. They approved as requested and proceeded to ask how many souls and fuel on board. Engine instruments were all normal throughout the entire flight and we did not appear to be taking on more oil on the windscreen. We flew direct Runway XX. Upon landing and exiting the runway; Ground asked if we wanted to stay in position or taxi back to our hanger. I elected to go back to the hangar. At the hangar; we were met with several emergency vehicles and the chief flight instructor. After shutting down the engine; I inspected inside the oil door to find that the oil dipstick was not properly secure. This was determined to be the cause of the oil leak.The pilot under instruction had confirmed that the oil cap and dipstick were secure during pre-flight. In the future; I will make sure to inspect the oil cap and dipstick myself before flight.