New O/O, Old truck what do I need to keep her in top shape?

I would change every fluid. All three boxes, oil, power steering fluid, ect. You never know when they were changed last.
As for a cooler, look under it and see if there are 2 big 3/4” lines going into transmission. If so that and a fluid change should do it.
I’ve got a pre emissions series 60 and change my oil EVERY 12-14k miles. NO EXCEPTIONS. Today’s modern oil has all the additives you will ever need so don’t bother with them. Keep the oil changed at regular intervals.
New filters are a must also. Fuel, oil, air, ect. Check for tranny filter. Some have them. Look in the tanks. I would pour in some PPpF Killem just to be sure NOTHING is in there. You may also be needing new fuel lines. Something to plan for.
Winter is coming fast so some new batteries should be in the plans as well.
Old steel is the best steel as you can work on it yourself without needing all that computer bs.
Good luck and don’t be afraid to ask questions.

 

before you change the oil, pull some samples from the engine and trans and have them analyzed, this will give you a baseline and tell you if there is any issues.

 

If you have a transmission temp gauge, monitor it to see how high it gets. With modern synthetic 50W 250 degrees and lower is not a problem. If you are running around local with your outfit you may never get vary warm.

@sbaumann14 gave good advice in my opinion on the rest.

 

My own personal experience with Lucas I wouldn't put that #### anywhere near anything I own.

 

I've used their hub oil without problems, but nothing else.

 

Just Engine Oil and filters every 12,000. In your application I might do trans and rears bi-annually - probably way more than necessary but oil is really relatively inexpensive compared to repairs. This will depend a lot on how many miles you run.

 

Any Cat dealer can set you up, and there are other companies that as well. Blackstone for one I think. I use Cat so am not familiar with the others.