I am going to do an oil change together with new filter and spark plugs over the weekend, what is the best oil I was considering using castrol power 1 10w40 semi synthetic ? also are NGK iriduim plugs any better than standard ones ?

Any A-brand 10w40 semi synthetic will do the trick (motorcycle oil). I have run for a long time even on fully synthetic Castroll R4 oil (5w40), with no slipping clutch plates at all. But many people have, so i say...just stick to semi synth if you want to be sure.