Cat Man HHR

I saw this on a recent closed tread and would like to explain something about it.
"I switched oil to the New Rotella T6 5-40 full Syn. and so far the gas mileage is about 2 better than Rotella T 15-40 which is not Syn and thicker. The thinner oil could be part of the increase in mileage and the fact its full synthetic. "

Both the synthetic and conventional oil are the same weight of 40 at an operating temperature of 210* F. The 5w or 15w in front of it is the spec at which it can flow properly at a colder temperature as explained here:
Motor Oil Viscosity Grades Explained in Layman's Terms

donbrew

It is probably the heavier weight has the oil pump struggling to push the sludge around and the crankshaft meets the sludge. And the VVT can't work correctly.