Stihl calls 50:1 "lean" - this does not match with your last paragraph, so there appears to be a disconnect in terminology.
It's common to refer to less oil in the mixture as a "lean" mixture and more oil as a "rich" mixture.
With that said, when the fuel gets into the carburetor, the less oil present, the more easily the gasoline vaporizes. That means that low oil mixtures, used with the carburetor adjustments, will result in the engine running richer.
When more oil is added, the gasoline vaporizes less readily. Unless the carburetor is adjusted to account for this, the end result is the air-fuel mixture being leaner.
Despite the higher oil content, leaner air-fuel mixtures will tend to burn hotter and result in hotter cylinder temperatures. It may be counter-intuitive, but that's the case.
Of course, you can adjust the carb to account for this assuming the carb is adjustable. If you go too heavy handed on the oil, you may run out of adjustment and be into the range where you need to rejet to be correct.
So, again, proceed cautiously messing with what the manufacturer recommends.