[William Miller]

Cap is the first thing to check.

Always check oil when hot.
My oil gauge actually seems to work. Every car is different, but I can watch the level on the gauge increase once the car is warmed up. If I was to fill it cold it will be near the top of the gauge when it's warmed up.

I don't know if this is because of expansion, or it has to do something with the oil circulation pattern as the engine T-stat opens to the engine oil cooler and then the aux stat opens to the front oil cooler.

AS GB83 stated the mixture is effected (leaned) when you open the cap. It allows unmetered air into the system. The engine slows a little when it is lean. On some occasions (models) it will tend to stall. All of this is dependant on the condition of the engine and how the maitanance has been. And O' yea tempature and humidity. All have an effect.

The 83 SC's (USA 930/16 engine) have a computer to monitor and adjust the mixtures for these variations (Closed Loop)
79 and earlier US as well as ROW up to 83 have the K-basic CIS which use more crude means to adjust mixture to compensate for tempature only and enrich the ratio under load.

Mine is an 83SC ROW. I installed air/fuel meter and can watch as ambient conditions effect the A/F ratio. I can also see how the mechanical parts compensate for tempature. When I first start my car I can see it running rich and lean out as designed. I can also see that it runs lean for a while as it goes from warm (say 60 C , the top of the white box) to operating temp 85 C when the thermostats start to open.) My system leans out a little too soon.

If yours is a K-lambda (80-83 US SC) I would think the computer would compensate (Richen the mixture) for small vacuum leaks as well as when you remove the cap.

If that's the case then I would suspect the Oxigen Sensor.
There are a few tests that will pinpoint it. Can anyone with a computer confirm this? I'd like to learn more, but I don't have a K-lambda car to play with. Maybe unplug it and see if anything changes?