Interesting mpg change with synthetic T6

[startrek]

My 2019 2500 4x4 cc 3.73 cummins had factory oil fill until 4k miles and I averaged 21.1 mpg empty interstate only
at 4k miles I took it for my free oil change and they put conv 15w40 back in it, I guess they don't read the owners manual, mileage dropped to 20.1 mpg
I could not stand the thought of the wrong oil in it so at 4.8k I change oil and filter using 5w-40 T6 syn and mileage is now at 22.1 mpg calculated

 

[Burn'n Oil]

You shoulda taken it back to the "free oil change" place (dealer, I suspect) and politely instructed them to drain the 15W-40 and refill (with a new filter) with an oil specified in the OM. If they'd refused, you'd request, in writing from the dealer GM, a guarantee that the power train warranty would not be compromised by operating the engine with oil that is not recommended by FCA.

 

[User_1648]

That is amazing mpg!! Is anyone else see that kind of efficiency with the 6.7L?

 

[Deleted member 80]

Is anyone else see that kind of efficiency with the 6.7L?

Only if mine is on a trailer.

 

[geektoad]

the oil likely had nothing to do with it. DPF regen during that timeframe is much much more like reason for the drop in MPG.

 

[elephantrider]

I'd get that magic oil back in there asap!

 

[startrek]

You shoulda taken it back to the "free oil change" place (dealer, I suspect) and politely instructed them to drain the 15W-40 and refill (with a new filter) with an oil specified in the OM. If they'd refused, you'd request, in writing from the dealer GM, a guarantee that the power train warranty would not be compromised by operating the engine with oil that is not recommended by FCA.

I considered that but they would want to do the emissions recall and that's not going to happen.