I run Opti-Lube XPD. There was a study showing it to be best lubricant additive. I run my Edge CTS Race on Level 3. With no additive; my mileage was 17.6 MPG covering 321 miles. With XPD at 256:1 my mileage was 19.2 MPG over 325 miles. That is only 1.6 MPG increase in economy. XPD costs $0.16 per gallon to treat at 256:1. ULSD with no additive is $2.599 per gallon or $0.147 per mile at 17.6 MPG; ULSD with XPD is $2.759 per gallon or $0.144 per mile at 19.2 MPG. I make this run at least three times a month when I'm home.
That is 3/10 of one cent savings.
I don't see myself running this much beyond the supply I currently have.