Try vortex.com. I put one on my series 60 (1998) when I first got it. Since I didn't have a base line I can't tell you what difference it made other than it increased my low end. It used to fall off below 1500rpm but now pulls good down to about 1300rpm. Not as good as a CAT but definitely better than it used to be. I think my mileage is pretty good. When I am just hauling dirt, I get close to 8mpg (of course the back trip is empty). I took a 250 mile trip up over a mountain pass pulling my backhoe on an equipment trailer and got 7.8mpg for the trip.
The vortex was originally designed for the series 50 (which is the 12.7 with the 2 back cylinders removed). It is used a lot in buses which run the series 50. Although originally used to increase mileage, it was discovered it also helped emissions - helps to get more air into the cylinder for cleaner burning and hopefully better mileage.
I found out about it from a friend that does diesel drags and sled pulls. He is always dynoing his motor and noticed an increase after installing one of the vortexs. They make them for most motors now. I dont know if they are effective on all of them, but is seems to be good on my 60.
I think I paid less than $70 delivered to my door so it is not a big risk if it doesn't work. The theory behind it sounds plausible.