FWIW, just sharing past experiences:
Fortunes were made every year years ago with engine oil level tricks before self-serve gas stations because after car drivers would pull up to a filling station, the attendant would pull out the dip stick, & because the entire interior of the engine had a heavy coat of oil, the attendant would show the guy that his engine's dip stick indicated a quart low below the "F" mark -- it worked with many, & quarts of oil were sold all day long like this.
Many rookies in the military flunked their first Military Inspector General (IG) motor pool inspections because of adding oil to a warm military engines whereby when they cooled, they registered above the full "F" mark. Engines were always inspected prior to start up & the oil level was expected to register full, at the "F" mark, when the engines were cold.
All it took was one (1) single ferocious butt reaming after flunking a military IG inspection for any military guy with any rank to remember "Military Oil Level 101" for the rest of his entire life; i.e., oil level should be at the "F" level when the engine is cold.
Hope this helps to remember the reason for "F" mark on the dip stick. LOL