I assume you have the C-13, 525 HP CATERPILLAR, engine.
It is recommended to do a PM at 16,000 miles. Since you do not have the manuals
Good idea to stop by a CAT dealer.
You will want to have the valve lash adjusted between 36000 and 50k. Good for the rest of ownership.
Personaly, I change oil and filters every time I make a coast to coast round trip. This keeps me in my personal 12000 mile limit. Take into consideration time sitting. Time also takes it's toll on used diesel oil.
It is a fact. As stated the best insurance you can get is a regular oil analysis.
I like CAT dealers (PETERSON IN EUGENE, OR they supplied all CAT engines to CC) for the occasional service and SPEEDCO for on the road quick PM's and oil analysis.