dino oil vs synthetic mpg

I've tried it several times over the years including my Cummins. never saw any measurable improvement in fuel efficiency.
The biggest change I ever saw was with my motorcycles. I had to lower the idle speed on those.
The improvement comes in the form of reduced wear so in most cases it's worth it. With my Cummins I wont use it. The eng will go many hundreds of thousands or 1 million plus on dino so that's good enough for me.

I have been using synthetic oil since 1974....

a few get 1 or 2 percent better.. but the heavier the car/truck Forget it.
just better protection..

I use synthetic pretty religiously. But mostly for the cold weather starting benefits. Never noticed any difference in mileage.

I think you are fooling yourself if you think it's going to be in mpg.

Years ago, I documented better mileage using Mobile One compared to then regular oil. There was a measurable difference in mileage. But not so much with modern dino oils.

Better MPG using synthetic is just advertising hype.