I currently run M1 10-30 in my 2001 2500HD with the 8.1 liter engine. I tow our 6000lb trailer a few hundred miles per month,but most of my driving is around town 3-5 miles at a time. I'd like to stick with a synthetic,but I don't like how thin the M1 seems. My manual says 5-30 or 10-30 is acceptable. Does M1 make an oil that is a bit thicker,than the 10-30,but is still ok for my truck? I live in W.Washington,and the temps rarely go below 32 even in mid-winter. Thanks for your help.