Are you sure GM specifies 5w30 for that engine? Even the new 3.8 engines out now all specify 10w30 actually. I wouldn't go with a 10w40 unless it's a full synthetic, as 10w40 conventional oils will thin out fast. Why is your mechanic saying you need 10w40 though? Is the engine burning oil? Just because an engine has higher mileage on it doesn't always mean it needs a thicker oil. If it's doing well so far on the 30wt oils, stick with it.