Yes, I really meant 100,000 miles. Mobil says WITH analysis AND filter changes, you can go indeffinately. I was paying $80 for the 4 gallon case it comes in ( 1 gal jugs is the smallest unit they sell in - 55 gal drums is the most common) So, today you're looking at about $28 per gallon. This is still a very good deal IF you go extended run.
I used Titian Labs - it was sold at K-Mart for about $12 a test.
http://www.titanlab.com/ You can order it directly from them, maybe some pro auto parts stores.
Both PowerStrokes are still running strong, but I don't have them.