C182RG instructor pilot reported taking off with the oil filler cap not installed.

Attributes
ACN	1350158
Time
Date	201604
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Skylane 182/RG Turbo Skylane/RG
Operating Under FAR Part	Part 91
Flight Phase	Initial Climb
Route In Use	None
Flight Plan	None
Component
Aircraft Component	Oil Filler Cap
Person 1
Function	Instructor Pilot Flying
Qualification	Flight Crew Flight Instructor Flight Crew Instrument Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 20 Flight Crew Total 7000 Flight Crew Type 500
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Procedural Published Material / Policy Inflight Event / Encounter Other / Unknown

Narrative:

CFI applicant was instructing me on proper preflight and systems. During the course of the lesson we took off the oil filler cap and inserted a funnel intending to put in 1 qt of oil. We determined that the oil level was sufficient for the flight (8 qts) and it would be unnecessary to add the additional quart. We removed the funnel and took the quart of oil back to the supply cabinet; returned to the aircraft and closed the access door neglecting to put the filler lid back on. During climb out I noticed oil streaming out from the access panel and realized that we forgot to put the cap on. We [advised ATC] and landed without incident. Upon removing the cowl at [the] ramp we discovered that we did leave the cap off. Less than 1 quart was lost according to the dipstick. Maintenance was called and the aircraft was taxied to them to have the engine cleaned. Without having a clear understanding of who was doing the actual preflight and who was instructing; we both missed that the cap was left unsecured.

Original NASA ASRS Text

Title: C182RG Instructor pilot reported taking off with the oil filler cap not installed.

Narrative: CFI applicant was instructing me on proper preflight and systems. During the course of the lesson we took off the oil filler cap and inserted a funnel intending to put in 1 qt of oil. We determined that the oil level was sufficient for the flight (8 qts) and it would be unnecessary to add the additional quart. We removed the funnel and took the quart of oil back to the supply cabinet; returned to the aircraft and closed the access door neglecting to put the filler lid back on. During climb out I noticed oil streaming out from the access panel and realized that we forgot to put the cap on. We [advised ATC] and landed without incident. Upon removing the cowl at [the] ramp we discovered that we did leave the cap off. Less than 1 quart was lost according to the dipstick. Maintenance was called and the aircraft was taxied to them to have the engine cleaned. Without having a clear understanding of who was doing the actual preflight and who was instructing; we both missed that the cap was left unsecured.

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.