Don't let anyone confuse you, any quality, diesel rated oil will do the job - look for the rating , CJ4 ...
Get the best price you can find and forget about brand...
Now having said that, I tend to be partial to Rotella-T 15W40... I use it in the boat, and the tractor, and the back hoe and the bulldozer, etc... I buy it in larger quantity for the price break...
As far as changing your own oil and filter, you should... No one will care as much about your engine as you will... Get the filters from Andrew (do NOT go to a Cummins dealer, they will rape you)... Put a hook and eye on the step so it will stay in the up position while you work... Remove the front board under the step - 4 screws... Now you have room to work...
I have a suction pump for removing the oil - found at auto stores, West Marine, etc.. It has a scavenge tube that slips down inside the dip stick tube... This works best with the boat on the trailer so you can get it nose down slightly to scavenge the most oil - but will work in the water with the stern low, you just don't remove all the oil (no biggie)... The savings from your first oil change will pay for the suction pump...
Any questions, just ping this group...
cheers