When I went thru section called "Maintenance Schedule" in my manual they say "Severe Duty" usage is when vehicle is operated in a dusty and off road environment or is operated predominantly at idle or only very low engine RPM's. And you should replace oil every 4,000 miles. They also explain that the truck sensors knows what is the outside temperature and if you are towing something or driving like crazy and their complicated algorithm system calculates exactly when the oil should be changed. So I'm confident if I follow the OLM I should be good to go. Of course if you like to do it often I won't blame you. I totally understand. But me myself is this kind of person who doesn't like to give someone my hard earned money.