I've always been sort of puzzled about this myself. Here is the paragraph from the Haynes manual for Vespa which troubles me:
"Do not on any account add extra oil to the petroil under the mistaken belief that a little extra oil will improve the engine lubrication. Apart from creating excess smoke, the addition of oil will make the mixture much weaker, with the consequent risk of overheating and engine seizure." (p. 26)
I don't know if it is correct or not, so I pretty much stick to the 50-to-1 ratio and mix 4 ozs. to 1.5 gallons of gas.