Let's imagine a situation whereby the oil filler cap was left off. Then the engine was started, warmed up and the run up checks completed. After rolling with revs for take off and fuel flow normal, wouldn't the oil pressure gauge identify the cap being off?
Unlike some of the posters here I haven't started on the good Red at this hour of the morning so this is a serious post.
My experience occurred in a C210 and given that the engine compartment in pretty much all of them is the same I reckon their reaction to an oil cap left off would be similar to my experience.
Usual story - rush ,tear bust to get going. Luggage to deal with, oil to be topped , oily funnel to be wrapped and stowed before replacing the oil cap because it was a dirt strip and windy so the funnel couldn't be put on the ground. Distracted by sendoff people while walking to finish the job and then quickly shut the top cowl door AND totally forgot the cap.
All was normal through start up and 1,000 m backtrack and mag and prop checks ( done on the backtrack to protect the prop on the stony dirt strip. All was normal on the take off roll and then I rotated. Almost immediately the wind screen began to be covered in oil. After the initial shock I guessed what the problem was and lowered the nose as much as I could. I climbed to a dizzy height of 300 feet and reduced power and speed as much as I dared. I found a small piece of the screen not completely covered in oil and completed the circuit.
After stopping I found that indeed the cap was off and was hanging from its' chain. Although there was a #*&%!% awful mess I had only lost about 1 litre of oil. In an hour or so enough oil had been cleaned up to be able to fly. The rest made its' way out over the next 2 years but on the positive side I never had cowl corrosion problems over that time.
I can't say what caused the oil to cover the screen on rotation. Possibilities that come to mind are:- change of sump angle or change of air flow over the open filler pipe causing suction there or there was an oil build up the whole time and the climb airflow redirected it to the windscreen. Whatever, the shortest, lowest, slowest flight possible still seems like the second best choice.
The best one would have been to not leave the cap off in the first place.