I have discussed several times the unsuitability of modern oils in classic cars and bikes with their big flat tappets and larger running clearances etc. The problem is mainly due to the lack of zinc in the oil, which for our purposes needs to be at least 1200ppm.
We have now started carrying this engine treatment additive from Hipertech (Part No H100350 for a 350ml bottle). Adding just 35ml to each liter (that’s about 100ml for the average oil change) will bring the zinc level in your regular oil up to at least 1500ppm.
The product’s primary benefit is to reduce friction thereby reducing wear and ensuring long and reliable engine life. It also helps by protecting against viscosity and thermal breakdown, enhances rust protection (important in vehicles which go for long periods without starting) , improving power and compression and lowering oil consumption.
Mike's British Motorcycle Thoughts 
Well that sure sounds like a lot of hassle. Canadian Tire and Walmart both sell Castrol Grand Prix 20-50 motorcycle oil which is ideal for classic British bikes.
Regards, Simon