I am using mineral oil in my scooter XC500. It's an high quality 10w/40. Have used mineral oils in almost all my bikes and I must say they are awsome. Did a "test" on my 125cc underbone Bashan, Chinese made Honda Innova, and I drove it quite hard in 11000km with no more than 2000km between oil services. This model have no oil filter... When I sold it the engine was smooth as silk, I checked valve clearance, clutch and cam wear etc. But nothing.. when I did the valve checks I noticed how thick the oil film was on the rocker arms etc. Never going back to syn again. If you have an single or a twin bike go for mineral.
Don't get me wrong, I understand that you can get better test result in a lab with an syn oil and some engins require syn just because they have so tight fit in the engine (tolerance), even lower mpg, higher power etc. BUT... does your low powered bike need all this? A fiend of mine used to race his Suzuki 1100 with god who knows how many hp. With more tha 80000km on the dial. An other with 800hp car-only mineral oil.
What type of oil are you using?