oil is marked in viscosity 10w means watery oil for winter starting if you like (easy starting less resistance for the engine) when oil warms up it thickens to protect the engine to a new viscosity,30 to 40 is not a big issue when engine is hot,i have never had an issue with Haynes as they check all of there data with manufacturers i would be quit confident to go with their recommendations.Hope this helps.
Rudy, October 2008
