valkraider wrote:
How would the weight affect mileage, once the vehicle is warmed up?
If the oil is a 0w40 or a 15w40, either way at operating temperature it would be like a 40w.
Wouldn't mileage be more affected by the other properties of the oil, like additives or whatnot - more than it would be by the weight.
Again, assuming the vehicle is warmed up...
It seems the mfg's found about 3% to 6% better fuel mileage when going from 10w conventional to a 0w synthetic. Now to you and I that small savings do not seem worth talking about and in some cases we could not even tell the increase, but when measured across millions of auto's it is a huge drop in gas used and oil import needed. Just think the effect we are having with the CRD and our average 30% better fuel mileage. Also there was a big fuel savings in winter starts and warm up with the 0w synthetic over the petro 10w oil common used for the last 3 decades.
The wt of oil does make a difference in mpg's and in a non synthetic with wear it gradually looses the ability to be a 15w-40 and by oil change viscosity may be down to 30 wt or even 20 wt, however if oil is over heated it starts to thicken and then starts forming sludge. Mfg's have had a lot of engine problems with some models in the last 10 years and most due to owners and mechanic's/service managers not understanding the importance of using oil required by the auto maker. Sometimes the manufactures were late finding out this problem, Chrysler & Toyota come to mind here.