correct engine oil for my 2006 2.2 litre zafira?

On the service pre delivery of the car to me it shows the oil used to be a 10w-40. Could the 10w-40 have caused the car to sound like a diesel after only 25,000 miles? My garage have now replaced it with a 5w-30 fully synthetic and we will see what happens but I just wondered whether there is an expert's view on this?