Your seasonal temp ranges should dictate the weight of oil. Although 15w40 is the most common I personally recommend 5w40 for year round best performance. 5w40 is a full synthetic and higher cost but it is possible to extend out your drain intervals to offset the increased price.
There is a thread on here someplace where we talked about synthetic quite extensively. I have changed over to a synthetic 5w40 that has free (read: added into the cost) oil analysis. I have no trouble getting 8k on route buses and 10k on those traveling mostly highway miles (over 150 to 200 per day). I bring them in at half miles for a look over and grease.