Few people take time to reason things out as you have here. I think you make good points and I'm responding more so for those such as yourself who rely on evidence to help them decide.Sometimes manufacturer specifies, but when in doubt I use unscrewed measurement for a couple reasons:
- It’s more conservative. More oil. I’d rather take my chances that sump can be overfilled by the “difference” than having less oil available in case engine starts burning or leaking oil. Some may argue that overfilling is a greater danger.
- I never believe the first measurement. Especially if engine has been running. I always wipe the stick off and re-dip. Thus the unscrewed measurement is easier, and “the way it should be” with something well designed.
And whatda you know,here's a perfect place to start. The old Briggs plug is a foolproof way of maintaining perfect oil level. Many others have used similar set ups,pitcocks on side of pan on tractors is one. More expensive engines and those that would spell disaster should they fail (think marine) took it one step farther by incorporating devices that shut enging down before engine starved for oil. Many if not most used dippers to lub crank and splash an approperate amount in cyl wall and other components. Oil level was very critical,too low and dipper never entered or partly entered oil,too high and crank beat oil into froth,not to mention impact of jurnel stricking oil (slap water in swimming pool to experience impact). Frothing limited oil to crank and spray found it's way out vent and evac tube. If all that's clear as dirty oil,I'll try making a point related to origional subject. A pint of oil added to a shallow 100 sq inch crankcase such as a single cylinder engine might raise oil on stick by 5/16 whereas same pint in 50 sq in pan might raise oil mark by 9/16. By volume,where one pint raises level 9/16 on last engine,it would take near twice as much to raise first engine 9/16 on stick. Short sticking(cap unscrewed) a 16 quart reservoir has far less impact than short sticking a 5 quart reservoir. I think,,,doesn't it,,,,,maybe?Because of some of the problems mentioned above, I liked the old styled Briggs engine where you removed the plug and looked down at the oil better.