Just had to rebuild my rear cylinder and wa wondering what people best choice of oil was, reading lots of stuff about clutch slip using fully synthetic but want to use the best oil possible, what's you're choice people??

Don't  get too hung up on oil. Its all pretty similar. 

Any motorcycle oil will be fine in the 650. Your hardly stressing the oil in that engine. You will be changing it regular anyway.

Its the car oil , motorcycle oil bit thats important.

Car oil has addatives that can ruin a motorcycle clutch. Also motorcycle oil can cope with the stresses of being used in a gearbox. Not gauranteed with car oil.

Mine got on fine with semi-synth 10w40.

Brand of your choice/budget, really. So long as it says Motorcycle Oil and is for 4T/Four-stroke, you'll be good.

One of our members works for  OPIE oils, here's the site address http://www.opieoils.co.uk/default.aspx