I change mine at the recommended 7500 mile interval. I do know one thing, the oil goes black real quick with the 600 compared to my 03 SO.

 

I've decided to err on the side of caution and change my oil and fuel filters every 8000km (5000miles) The local cummins dealers sells nicely prepackaged kits for the Dodge trucks with all necessary parts to make it easier, with tax its $105.00 Canadian for fleetguard filters and 15 litres of Valvoline ultraguard blue 15w40 oil. I talked to 3 different dealers and the cummins dealer, got 4 different answers on oil change intervals, so I came up with one that seems reasonable to me.

 

You have to do what makes you comfortable. Some may say it is a waste and others may say it could not hurt to change early. If I towed a lot I would do as you said and change earlier also however I dont and I feel comfortable changing at the recommended interval.