Re: Oil change int.
Why are you using a 10W-30 viscosity? 15W-40 is the recommended viscosity this time of year. Keep in mind that higher viscosity oils tend to give you better protection, particularly when towing in summer temps.
You can't judge your oil change interval by the color of the oil. Your PSD has a completely different fuel injection system and combustion characteristics than your lawn tractor.
You could have the oil analyzed to see how the engine is dealing with your interval, but 5K miles towing is probably a good time to change it. I would. PSD's like fresh, clean oil. FWIW, Ford recommends 3K miles under your conditions, 5K miles under normal conditions.