Originally Posted By: motor_oil_madman
That would call for an oil change every 3000 miles on my cummins.
Quite a bit of difference between a baby diesel in a pickup compared to a 3000 lb 15L working engine in big truck. The Cat manual on the C15 calls for 25,000 miles / 500 hrs for normal service. For severe service, the manual suggests 250 hrs. It recommends oil sampling at these intervals also. For the OP's use, 250 hrs would be a good starting point and you could work your way up a little each time and do sampling to see when things start to show you have reached the maximum length you can go. Change and do a sample at 250 hrs, then next time do a change and sample at 300 hrs, etc until you reach the point where things look like it is deviating from the trend pattern. I have typically taken heavy diesel engines to 50% longer than the OEM recommended drain interval and still been well within any safety margins. And that is on regular CJ-4 conventional oils.
Far cry from the newer engines. Some of them at least 35,000 miles with a few in the 50,000 mile / 1250 hrs category for OEM recommended oil changes.