Originally Posted By: Dohc98vteC
I am getting horrible mileage even for winter standards here. The only thing different is my dealer insisting on using 5w-20 or 0w-20 oil, instead of 5w-30 based on the Acura oil chart they have at the service dept. But the thing is for 2004 only 5w-30 is specified, not sure why same model same engine TSX would show 0w-20 and 5w-20. The car feels real sluggish too like performance is suffering. Bottom line, with temps here at well below 0 some nights, should I switch back to 5w-30?
Technically, yes, thinner oil could reduce MPG.
Thinner oil increases frictional loss in the valvetrain.
Thinner oil tends to decrease frictional loss in the bearings.
Thinner oil may increase or decrease the frictional loss in the cylinders.
Again, thinner oil always causes more fricton in the valvetrain. However, bearings and (most parts of the) cylinders are hydrodynamically lubricated and unless there is oil-film breakdown due to thinner oil, friction will be reduced. If there is oil-film breakdown due to thinner oil, hydrodynamic lubrication will fail and you will have more friction thanks to metal-to-metal contact like in the valvetrain.
So, yes, in principle, there is an optimum viscosity for highest MPG for a given engine. You can't go arbitrarily thin (0W-16, 0W-10, etc.) and hope that your MPG will keep increasing.
Also, different oils with give different MPG for the same SAE viscosity grade because of different friction modifiers and different base oil (synthetic gives better MPG).
All this said, I doubt that your loss of MPG is oil-related but I think you are more likely having some other maintenance or tune-up problems. Dealer might have messed up something else. Also, it's winter and MPG usually drops in the winter.
