Ok, here's the maintenance intervals on the older C13's & C15's:
http://safety.cat.com/cda/files/557611/7/3406E,%20C-10,%20C-12,%20C-15%20and%20C-16%20On-Highway%20Engines-Maintenance%20Intervals.pdf
Fuel filters are due at 15,000 or 2500 gallons or 300 hours. As stated above, these engines...