Which oil is better ?
Use either semi-synthetic or fully synthetic after the brake in period (first 1000 miles) It is a cheap insurance.
Depending of the time of year and your location make sure that you understand what oil you should use, the difference between a 20w-50 and 10w-40 is that a 20w-50 has a weight of 20 at 0° F and a provides the same protection of a 50 at 212° F ... weight could be understood as viscosity... while a 10w-40 has a weight of 10 at 0° F the same protection of 40 at 212° F. So, what is the weather like when you ride? How hot does your bike get when you ride? Choose accordingly.
Keep in mind that the oil does NOT gain viscosity as it gets hotter, it is just the resistance to thermal breakdown that change. Meaning that your 20w-50 will protect your engine as well as a 50 at 212° F but it has the advantage of being only a 20 viscosity which is good for average temperature morning in southern california (above 50° F).
Now here is where is gets interesting... Which one protects better a 5w-50 or a 20w-50? I like to think that most of the wear of the engine occurs when you first start the engine up, so the 5w-50 will protect better at start ups because of the lighter weight BOTH will protect the same at 212° F.
Fuel efficiency is also affected by the oil weight the lighter it is the less work is need to move it around but I am not to convince of the 0w-50 as the seals do need some weight to work properly, new bikes can go as low as 5w-X older bike should not go below 10w-X. Lots of information all over the internet you need an afternoon to go over all of it but I wanted to post my two cents worth of advice I hope it helps.
Luisimo.