Switched oil, loss in MPG???
 

Alright, I will go ahead and put my flame suit on, but here is my question. I was running Schaffers 5w-30 in my truck. THe place that I was ordering this from said it was going to be several weeks before they got it in. So i opted for Rotella T 15W-40. I have noticed that I m getting about 1 mpg less than I was before I switched oil. Is this possible. Pretty much same driving patterns. Or am I just a weirdo from KS?

when i went from 15-40 to 5-40 Rotella syn i did see smother running and faster rev up. i can see getting more drage with the heaver oil. so i would belive it.

Theoretically friction DOES play a role in mpg's. There have been several articles that discuss the oil weights and economy. Not just in the engine but also the drivetrain. The 2007 model trucks have a suggested oil weight of 5w40, and this suggested oil weight has been determined with emissions in mind.

CD

What you've experienced is called parasitic drag. You increased the viscosity across the board with the change in oil weight. It is thicker cold (5w vs 15w) and hot (30w vs 40w). The ability of oil to protect is based on film strength, not viscosity. You probably noticed a higher oil pressure reading too.

My 2006 owners manual shows that synthetic 5w40 oil is approve for use year round in my engine. I have Amsoil 5w30 HDD in mine right now an it runs quiter at all speeds then it did when it had Rotella 5w40 synthetic oil in it. I also saw better mileage when I quit using 15w40 oil and went to the lighter oil 5w oil so I'm not surprised you got lower fuel mileage going to 15w from 5w oil.

Depending on your driving cycle, the first or second set of numbers on the oil has more to do with the change. If you are driving short trips with a lot of cold starts, the 5w to 15w change will bring you down. If the truck is at operating temp most of your driving, the 30 to 40 switch comes into play. For a 5W40 oil, it is tested as a 5 weight oil at the cold temp test and a 40 weight oil at the hot test. A 5w30 that is diesel rated will give you the best mileage but may not be better if you load your engine a lot.