Like many classic cars, your classic bike will have been designed to run on leaded four-star fuel, which is both hard to find and very expensive these days. Your bike will run fine on unleaded, although you may wish to use an additive if you are concerned about valve damage or retard the ignition if you experience 'pinking'. Perhaps a bigger concern is the increasing use of ethanol in fuel. Ethanol is not kind to older components - rubber pipes and seals, zinc and brass. I understand that, with motorbikes, there is also an issue with the ethanol destroying the glassfibre resin and causing the fuel tank to go soft. Again, a range of additives are available to help deal with this problem. Just be careful if you plan to use your bike abroad - most of the fuel in Europe has a much higher ethanol content.