...then why the lower HT/HS numbers? I notice some oil companies who publish these HT/HS specs on HDEO dino 15w40, but when you look at that same companies specs on their synthetic 5w40 oil...they do not post that info/spec. The companies that did indeed post the HT/HS specs for 15w40 and 5w40, their "synthetic" HT/HS number is lower then it's dino 15w40 counterpart. Hence the reasoning I am sticking with 15w40 dino oil in summer, and 5w40 in winter in my duramax. I am NOT taking a chance on a very expensive engine running a 5w40 in summer while towing. Some of you may agree with me, and some not....this is what makes me feel better. I have read online that duramax oil temps can get severely high while towing, and if i understand correctly, as your oil temp rises your oil thins out more and more, so I would rather "thin out" a stout 15w40, rather than a 5w40, or a 10w30. I am not brave enough as "dnewton3" or "arkapig" to run a 10w30 in my duramax yet.....