That's right, gents. In our case, carbon based molecular density correlates with the effect of lubricity. That's why you don't use diesel fuel for engine oil as a replacement for 15W40. The 15W40 has a longer molecular (carbon-based) chain so it can fill and thus lubricate microscopic gaps in your engine better than a short chain molecule can.
Think of using a golf ball as a bearing versus a bowling ball. The golf ball has say 25x more rotations to equal one of the bowling ball for the same application. More friction (breakdown via heat) occurs the smaller the molecule is. There is also a drag coefficient and that is why you use 15W40 instead of 80W90 in your engine. Drag coefficient slams the molecules into each other, creating friction and heat and breakdown. I hope I explained that well without having a fancy fluid dynamics degree.
