Oil *underfill* and why it might be good or bad

Appology if this has already been posted, but is a new insight for me.

It just dawned on me what might be one of the factors giving substance to the claim of fuel efficiency improvement with engine oil level under the high mark (compared to when it is at the high mark or higher).

I thought about it when replying to another poster who was claiming that if oil is below the high mark on the dipstick insufficient lubrication may occur. My answer was that it won't happen this way as there is a oil pick-up way down in the oil pan that should not get dry even with severely low oil levels. But that running with more oil may help absorb "flash" bursts of heat due to peak engine loads for short intervals, like merging on highway or passing, etc.

At that point I thought, well, if more oil absorbs more heat then less oil has the capacity to absorb less heat. And that is the sole reason I can think of that gives better fuel economy in the Prius - the engine can warm-up faster. In fact, 1/2 quart less oil means 1/8 less energy is needed to warm-up the oil compared to a full 4 quarts fill. Is that enough to show on the fuel consumption tracking spreadsheet? I don't know, yet. If an EBH can help so should less oil is my thinking -

On a separate note, here are some photos of the Fram SureDrain valve installed on my '02 Prius and also show it "in action". No tools required oil drain (you still need tools for the filter replacement!). On this valve, the drain rate is actually much better than on my other car's valve. I can get probably a quart a minute out, which is pretty much "real time" performance (as opposed to about 15-20 minutes for the entire drain on my other car).

I started with 4 full quarts of oil + a new filter. Draining 0.5 Liters (just over 1/2 quart) brought the level to about 1/3 down from the high mark (was just about at the high mark before).