On my Cummins 6.7L I change my oil every 12,000 miles.

I pull a 28 foot gooseneck trailer and get 10 Miles Per Gallon.

With 12,000 miles @ 10 MPG that is 1200 GALLONS.

I cut my frame mounted spin on filter open to see how dirty it (and the front / motor - filter) were after 1200 Gallons.

Here is what I found.