Regardless of oil type, I typically run 32:1 in my YZ250. The NSR uses an oil injector, though, so I dont have to mix the gas. If I was running mixed, it would be around 24:1 for high-revving machine like that (this bike is like two 125's coupled together). Basic rule of thumb; the higher the revs, the more oil you should run. 250's should be between 40-32:1, 125's between 36-24:1, etc. The higher revving engines really need that higher quantity of oil to get really good life from them.
You will most likely have no problems running a good synthetic at 50:1, IF you arent riding the bike hard. If you are one of those guys who rides around bouncing off redline constantly, it is very likely your engine will not last long at all.
Also, tests have proven that the any ratio run less oil quantity than about 32:1 actually decreases horsepower.