Any good quality 20W/50 engine oil should be adequate but opinions vary, some preferring to use the cheapest brands and doing more frequent oil changes while others tend to go for the higher quality oil, although the use of synthetic oil isn't really necessary.
20W/50 viscosity oil is not as common as it used to be so you may have to hunt around until you find a supplier or possibly get it by mail order from somewhere like Holdens or any good motorcycle shop should be able to order it from their supplier.
http://www.motocycle.co.uk/productDisplay.asp?code=090.912I tend to use Shell Optimax myself in my older motorcycles as it supposedly has a higher octane rating but other classic riders have used normal unleaded without problems.
There are also differences of opinion concerning fuel additives as some riders haven't used them yet have not suffered valve seat problems. I do normally use Castrol Valvemaster Plus myself but I may be wasting my money?
http://www.castrolclassicoils.co.uk/valvemp_new.htmI don't know where you live but leaded petrol is still available at certain places around the country.
http://www.leadedpetrol.co.uk/list.htm
