5-30 vs 15-40 oil how much MPG loss??

Better check to be sure that 10-40 oil didn't void your warranty. I now have a '15 cascadia w/ detroit d. that requires 10w30 only. Anything else voids the warranty. It has the 50,000 oil chg interval btw, so I guess is why it makes such a big deal with mine. Getting 8+mpg is excellent too.

 

The question is it really the oil? These days most oils are designed for performance 5-30 is just better for starting in cooler climates than 15-40 and so if you use a heavier oil it will just mean more wear on the engine over the long term, however there are other things that could be causing you to lose .3 mpg such as driving speed, hilly roads, strong winds, heavier freight etc etc.For every extra mph a truck goes you lose .1 mpg in fuel economy.

 

Ok well if the road is the same and the freight all the same then i guess it must be the oil well, the only way you'll know for sure is next oil change put the 5w-30 back in again and see if your fuel economy picks up again that way you will be 100 % sure it is the oil.

 

I would be looking for something from the dealership. Not having the oil recommended for their own brand of trucks is nonsense.

 

It would not be the oil that voided the warranty, if that indeed may be the case, it is the dealership who voided said warranty by installing the out of spec oil. Never let go of that service receipt!