Two vent holes

[Avanti]

Merc 60 hp 1.83:1 lower unit
1C558025

My lower unit has two vent holes and a drain hole.
The service manual, though inconsistent, describes how to fill the gear case.
My buddy's circa 1989 Merc 115 2+2 has two vent holes and I've filled that in the past the same way, as per that models manual.
My last merc's manual directed to let one ounce of gear case oil to drain out before securing the fill plug "to permit expansion of lubricant".
My current 60 hp manual advises to make sure less than 1 ounce of oil drains out before securing the drain plug.
What I am wondering is: are the two vent holes usable as a top off access, pushing oil in one vent hole and allowing excess to overflow out of the second vent hole while the drain hole is plugged ?
If it is crucial that the oil level be close to full. Although the manual does not imply that is necessary or recommended.
I'd always thought, due to mental laziness, all of the gears and bearings in the lower unit are just immersed in oil.
But looking at my parts schematic I finally realized that the drive shaft upper bearing is not, and seems to be the at-risk component in the lower unit.
I am kind of fascinated in the concept that oil somehow climbs the drive shaft or the sidewalls and reliably lubricates that bearing.
I see in the manual for my last motor a helical oil sleeve that must assist in directing the oil upwards, but I can't see the detail in my current drive shaft design that accomplishes that.
I found some patents via google, especially one from about 1970 by OMC that describes a non cylindrical drive shaft that accomplishes this using centrifugal lubrication.
But it almost seems like a common knowledge fact of nature that oil will climb a spinning shaft and/or the sidewall.
I would love to find a semi technical article on how and why this happens.
I also wonder if this design concept is used in any other types of machines.

[EuropeanAM]

The extra hole at the top of that gearcase is essentially that- "a hole with a plug in it". It should not be used as a vent or a fill point- utilize the LEVEL screw and the DRAIN screw. Fill from the bottom and correct level is "just at the bottom of the threads for the level screw". NOT overflowing or spewing out.

Install top screw with new seal, then bottom with new seal. Most manuals for this engine size that mention allowing some oil to drain are assuming the "full to overflowing" status that often occurs.

Note: Your question about distribution of lubrication brings into play a number of gearcases that aren't even in the same manufacturer line. Interestingly enough, back in the vintage of the OMC you referenced they even used HYDRAULIC pumps and Electro-Hydraulic shift mechanisms (gearcase defaulted to FWD when all else failed). Crazy stuff!

A number of gearcase designs actually depend on the oil that is being forced off a gear to move oil through passages at low pressure (some use this method to lubricate the upper crank bearing from the Forward gear, for instance).

Tons of variations.