Which Oil?

Can anyone tell me what difference it would make to my bike (88 GPX750R) if I used a different oil?
At the moment the mechanic put standard mineral based oil in it. If I was to go to semi-synthetic or synthetic would it make it run any smoother or would it run worse?
At the moment the mechanic put standard mineral based oil in it. If I was to go to semi-synthetic or synthetic would it make it run any smoother or would it run worse?
