Can some tell me which oil viscosity is correct if the 20w/50 don't sell it anymore what is the next level 20w/40 or 10w/50??

My scooter have a wet clutch automatic I don't want to add wrong oil to damage my gearbox. I live in Essex the weather is fair cold in winter and a very warm in summer. Which viscosity is the best all year?

Thanks. Fab