Not much, really. Clean fuel, which any adequate stock 2 filter system will deliver unless your tank has problems or you take on bogus fuel. Enough fresh air for combustion, clear exhaust. Coolant change every few years to prevent problems in the cooling circuit. New or rebuilt injectors when you're blowing plumes of black smoke(injectors go a long time without problems). Replace the few parts that wear out like water pumps when they fail.

Outside of this simple stuff, I've adjusted valve clearance. I wire brush and paint areas that need it, keep it clean, fix leaks, easy stuff.

Isn't the beauty of the design of a diesel that it is a simple robust machine capable or running a gazzillion hours with the above, fairly simple care?