There is an awful lot of nonsense around the net concerning 2T pre-mix ratios, and little understanding of the fact that dual purpose oils (which can also be used in 2T oil injection systems) are far from ideal for use in pre-mix competition bikes!
The ratio used is largely dependent on the heat being generated by the motor. An example of this would be a trials bike being run on 100:1, and a road racer on 30:1. Obviously the road race motor is running far higher rpm, and gets much hotter than the trials bike.
One of the main reasons for using more oil in high heat applications is due to more oil helping with heat dissipation. In some cases road racers will increase the amount of oil for courses with long straights, where motors are held flat out for longer, and reduce oil on tight twisty courses with less high rpm running.
In all cases though less oil will mean cleaner running, better pickup from low rpm and more power. However less oil is not a good idea if its a high heat application such as road race or MX. Using more oil for trials and recreational riding, will mean reduced power, less responsive motors and exhausts which become badly clogged very quickly.
Best possible oil to use for any pre-mix competition bike is a product specifically designed for this purpose, and not one of very thin dual purpose injector oils, whose use on comp bikes which may be used infrequently is far from ideal, purely due to the very thin injector type oil not being very good in terms of providing good corrosion protection when bikes are not being used.
Using full synthetic pre mix only oil, for air cooled trials 70:1, for water-cooled trials 100:1, for recreational off roading 60:1, for serious MX racing 40:1 will work fine. If you choose to use dual purpose injector type oils, increase amount of oil by 20%, as these oils are generally thinned by the addition of kerosene, so are a lot less oily than proper competition lubricants.