I seem to recall that way back when I first used mowers, the standard fuel/oil ratio was 16:1. Oil got better (2 stroke oil instead of engine oil) and engine manufacturers moved to 25:1. Some engines now are tested and approved by their manufacturers at 50:1, provided the oil is even better. These changes are arbitrary, dictated more by the environmental regulators and the marketing department than by the engineers. The regulators and marketers do not mind, within reason, if the life of the engines is shortened, and they would regard a 30 year old full crank Victa as a failure by the engineers who designed it: those engines denied them the opportunity to make more headway with environmental pollution reduction, and to sell more mowers.
So, how much oil is best? Well, how long should a mower engine last? Personally, I doubt that a 25:1 engine would last measurably longer at 22.7:1, unless there was a known design weakness in the engine that resulted in poor lubrication of some part of it. The extra oil will shorten the spark plug life, and make the exhaust smell even worse than usual.
