There's been mornings here in Texas in the last month, in summer, where it was below the minimum temperature recommended for that thick of an oil. I would only recommend going that thick if you have a good reason for doing so. Otherwise, blindly listening to a mechanic telling him to run 20W-50 just because it's a motorcycle probably isn't sound advice.I really don't think it has anything to do with the viscosity. 20-50 is fine, especially in a warm environment. If it's cold then it could be a problem. Was there anything else done to the bike while you dropped it off?
For reference, page 9-1 in the manual