Narrative:

About halfway during the flight in cruise flight at 9000 feet. A passenger tapped me on the shoulder and called my attention to the left engine. What I saw seem to be a fair amount of what was later described as a gray oily smoke or fog trailing from the engine. The passenger also mentioned the oil pressure gauge (it was later that I found out this passenger was a pilot) I looked at the oil pressure gauge which was in the green. My first thought was this was a contrail from the warm exhaust hitting the cold air; then I noticed the oil temperature gauge was dead reading 0 below the green. The other gauges on this engine and the other engine were normal. I tried to visually check the right engine; but it was difficult to see from the left side of the plane but with passenger assistance it was determined that there was no smoke/fog trailing from that engine. I ruled out the idea of contrails because it would be coming from both engines as I have seen before on very cold days at altitude. There was no engine roughness or loss of power on the left engine. I was not sure if it was a faulty gauge or a problem with the engine. I checked for a popped circuit breaker then checked through our quick reference handbook for any procedures for that problem; I did not find any that related to my issue. Again not sure if it was a faulty gauge; a problem in the engine that I might lose soon; or just cold air contrail; but I did not like the look of what I could see trailing from the engine; so I decided to get on the ground and get it sorted out. The passengers were also concerned with what they were seeing too. I was about midway between ZZZ and ZZZ1. I felt ZZZ offered options for maintenance; other aircraft; options for the passengers and services if needed. I contacted company and informed them of my situation and intentions; they confirmed my decision to return to ZZZ. I alerted ATC to my decision to divert to ZZZ they vectored me to the airport. Once in contact with approach they asked me if I was declaring an emergency; I said no; because the left engine had not shown any issue with power and ran normally all the way and I still had total and complete control of the aircraft. I flew the approach and landed normally with no issues and taxied normally to the gate and unloaded the passengers. As I approached the runway to land I saw that the tower had the trucks waiting to roll at the runway. I was unaware until I landed that ZZZ tower had issued the trucks rolling prior to landing.

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Original NASA ASRS Text

Title: Cessna 402 pilot became aware of smoke or fog trailing from the left engine and an inoperative oil temperature gauge. The Captain elected to return to the departure airport.

Narrative: About halfway during the flight in cruise flight at 9000 feet. A passenger tapped me on the shoulder and called my attention to the left engine. What I saw seem to be a fair amount of what was later described as a gray oily smoke or fog trailing from the engine. The passenger also mentioned the oil pressure gauge (it was later that I found out this passenger was a pilot) I looked at the oil pressure gauge which was in the green. My first thought was this was a contrail from the warm exhaust hitting the cold air; then I noticed the oil temperature gauge was dead reading 0 below the green. The other gauges on this engine and the other engine were normal. I tried to visually check the right engine; but it was difficult to see from the left side of the plane but with passenger assistance it was determined that there was no smoke/fog trailing from that engine. I ruled out the idea of contrails because it would be coming from both engines as I have seen before on very cold days at altitude. There was no engine roughness or loss of power on the left engine. I was not sure if it was a faulty gauge or a problem with the engine. I checked for a popped circuit breaker then checked through our quick reference handbook for any procedures for that problem; I did not find any that related to my issue. Again not sure if it was a faulty gauge; a problem in the engine that I might lose soon; or just cold air contrail; but I did not like the look of what I could see trailing from the engine; so I decided to get on the ground and get it sorted out. The passengers were also concerned with what they were seeing too. I was about midway between ZZZ and ZZZ1. I felt ZZZ offered options for maintenance; other aircraft; options for the passengers and services if needed. I contacted Company and informed them of my situation and intentions; they confirmed my decision to return to ZZZ. I alerted ATC to my decision to divert to ZZZ they vectored me to the airport. Once in contact with Approach they asked me if I was declaring an emergency; I said no; because the left engine had not shown any issue with power and ran normally all the way and I still had total and complete control of the aircraft. I flew the approach and landed normally with no issues and taxied normally to the gate and unloaded the passengers. As I approached the runway to land I saw that the tower had the trucks waiting to roll at the runway. I was unaware until I landed that ZZZ tower had issued the trucks rolling prior to landing.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.