I have a 2011 Triumph Tbird 1600 and the owners manual specs Castrol Power 10w40 or 10w50. The dealer mechanic recommends and uses the 10w50 exclusively but Advance Auto carries the 10w40. I would like to buy the oil at AAP for $9 and avoid the $15 dealer cost if possible. Does anyone know if there is much difference with the two? The engine is stock and doesn't get run hard at all.