A Line Mechanic reports he thought he saw adequate oil on the sight glass for both engines; but apparently mis-read the oil quantity indication for # 1 engine. During Take-off; the ATR-42 Flight Crew received a CAP warning and low oil pressure immediately after rotation.

Narrative

Weekday evening; when the ATR-42 came in from ZZZ; I checked the oil in both engines. I thought I saw an indication of adequate oil on the sight glass on both engines; but specifically #1 engine. Apparently I mis-read the indication; however; since when the plane attempted takeoff two days later in the morning; the Flight Crew received a CAP warning and low oil pressure on the guage immediately after rotation. They were able to set the plane back down and stop safely in the remaining runway. Immediately after landing; the oil pressure went back up.After the plane returned to the ramp; I could see no oil indication on the sight glass. I added 5 quarts of oil to bring the oil level back up. There were no signs of leaks on the Nacelle; the Prop; or in the Exhaust. The plane flew to ZZZ with no difficulty; but on landing oil was seen on the bottom of the Nacelle and a loose fitting was found on the oil/fuel Heat Exchanger.I will be more cognizant of the necessity for insuring proper reading of the engine oil level sight glass.