A couple of quick thoughts:

How much water was in the oil? If the truck had been neglected, and the oil not changed in a long time, it could be condensation buildup and a simple oil change could solve the problem.

Hard starting when hot can be an ignition problem. If the plugs are worn, start by replacing them and making sure the gap is set properly. Also inspect the wires, cap and rotor. It will be easier if you take care of maintenance issues before proceeding with a diagnosis.

1988 Isuzu Trooper LS
2.8l Isuzu 4JB1-TC intercooled turbo diesel
www.dieseltrooper.blogspot.com