How often do you change oil on a Cat3126?

How often do you change your oil on a truck with a Cat 3126? I had heard reports that if you don't change it every 5000 miles you are asking for trouble and they wont run right. I just picket up a truck, and had a hard time making it home. It was a trukcking co that owned it and they said they change oil in theirs every 15K miles and this truck was due for a oil change. I had to drive it 800 miles home. On a week end, couldn't find a oil filter, TA, Pilot, Flying J, Loves, none of them had a oil filter. Truck run like crap,, no power, had to downshift on smallest incline and some times would only run 45. Finally did find a couple of fuel filters and changed them but it didn't help. Will change oil in a couple of days. Shure hope that it will solve the problem.