Engine over full, diesel in the oil, excessive blowby.
There are several things that can cause diesel fuel to get into your crankcase. A broken piston ring is the most common cause.
Head gasket & oil cooler
Head gasket & oil cooler
Diesel fuel or coolant entering the crankcase. Either will cause serious engine damage. Stop driving the truck until you can have this repaired.
Fuel or air.
The most likely cause of low-power, when under load, in your Cummins N14 diesel engine is a lack of fuel. The problem can also be a lack of air flow.
Normal cause is a failed TPS. (throttle position sensor)
It is the high temp and the high return Pressure inside the injector that causes the fuel To turn black
A very likely cause is a failed radiator fan.A very likely cause is a failed radiator fan.
The turbocharger turbine housing and exhaust manifold on any state of the art high horsepower (per litre of displacement) diesel engine will glow red as it approaches full load condition.
There are several things that can cause low oil pressure on your comments 5.9 liter diesel engine. The most common cause of low oil pressure, when under load, is a faulty turbo.
Have the Cummins computer checked for trouble codes.