My best guess is about a 10% increase in MPG. As far as longevity of the engines, only time will tell but it does seem we are having less problems with our troublesome 6.0 Ford Diesel (known no to be the best Diesel Engine) it really appears that injector replacements are not as common in the engine as they were prior to using K-100 additive.
For me personally, I drive a F-250 6.0 Diesel, I am seeing a 12% increase in MPG when not towing a boat and about 9% increase in towing MPG by using the K-100 additive.
It really gripes me to say this: But Phil thanks for your persistence on me trying K-100 additive, it pays for itself in increased MPG and it appears maintenance cost is down also. Time will tell more about the longevity of engine life, but we have several 6.0 Diesels with over 300,000 miles still running good. Officially, I think the K-100 is a great investment.