Flight crew reported receiving a engine oil low pressure notification during flight due to a missing oil cap that was inadvertently left off after the preflight inspection.
Synopsis
Flight crew reported receiving a engine oil low pressure notification during flight due to a missing oil cap that was inadvertently left off after the preflight inspection.
Narrative
During the preflight early morning I delegated checking the oil to my First Officer (FO) while I did the rest of the preflight. As I was walking around the plane he was in the process of doing the left engine oil. When he was finishing; I went to hook up the tractor to pull the plane out. After the plane was out; we unhooked it; and my FO drove the tractor back to the terminal and I started to the plane to warm it up. At this point I didn't see anything abnormal outside since it was still before sunrise and it was dark in that side of the airport. Once in the air; as we were 5 miles from ZZZZZ (approximately) I started noticing the oil pressure fluctuate for the left engine and mentioned it to my FO. After a few minutes it dipped down to the yellow and it's at this point I decide to turn around and look at the engine and saw oil coming out more than the normal amount. After a discussion with my FO we decide it's the best course of action to get close to the shoreline as soon as we could. My FO then turned and told me that he thinks that he didn't get the oil cap back on. At this point I got a bit more relaxed knowing it wasn't an oil leak that was fully going to drain the tank. Still we asked to get a more direct route to the airport. Once we were abeam the airport at 7;000 feet the oil pressure dipped a couple times to the red. We decided to [request priority handling] even though engine temperatures were still normal. At this point we got priority on the approach and landed the aircraft. Once on the ramp and with the engines off my FO told the passengers what happened. My FO got on the ladder and did see that we didn't have the oil cap in. Not going up and checking afterwards that my FO got the oil cap back in. And finished the preflight in accordance with checklists.Verbally remind my FO if that task has been delegated to him; and then have me go up and visually check myself. If I'm the one checking then verbally confirming myself that the oil cap is in.
Second reporter narrative
During cruise flight for Aircraft X to ZZZ; 5 miles from ZZZZZ intersection Captain noticed the left engine oil pressure gauge was showing a fluctuating drop from normal (green indicator) operating oil pressure into lower (yellow indicator) operating pressure. Captain then looked at the left engine and could visually see oil leaking out of the engine. At this time I; First Officer; realized that during the preflight in the morning when I added a quart of oil to the left engine I did not put the left engine oil dipstick back into the position and secure it; and the dipstick was left on top of the wing and eventually rolled off onto the ground so the flight was conducted without the left engine oil dipstick. Captain and I; First Officer; monitored the oil pressure continuing to drop slowly; requested to fly more direct towards the destination airport. As the flight path reached abeam the airport (ZZZ) the oil pressure went into the low red indication area at which point Captain [requested priority handling] to be given priority to land safely. [The cause was] not concluding an in-depth preflight of the aircraft.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.