RE: STIHL oil ?
Everything else remaining equal, higher altitude will require a leaner mixture.
80:1 = 80 parts gas to 1 part oil, 50:1 = 50 parts gas to 1 part oil. By volume the latter ratio is a leaner fuel (gas) mixtrure, you would likely need to richen your needle settings slightly but a fuel/oil ratio is not the primary reason for the necessity. All oils are not the same, some contribute energy to the combustion process, some don't. Always, always, always, readjust your needles when changing oils and or oil ratio.