Oil and Excess Pressure in fuel

[HAULNTX]

Good evening, I am new to the forum.

I have a 09 Kenworth Cummins ISX 871 with a little over 300K miles on it that is getting what I think is oil in the fuel. The fuel tank builds excess heat and pressure and the diesel in the tank is very dark. Truck eventually shut down on us on the side of the road which started this project about a month ago.

Mechanic stated that there was oil in the diesel which I had not noticed. He thought it was the injectors. Once at the shop pulled injectors and realized the head had a coolant leak into cylinder 6. Replaced the whole head with a new one. Put everything back with original injectors. Couldn't get it to crank. Through researching this forum narrowed it down to the gear fuel pump. Replaced that and it fired right up. Ran well for about 10 days then the oil and pressure in the fuel tank came back. Figured maybe it was the injectors and replaced all 6 of those with no change. Today was the first day on the road again since replacing injectors late into last night. Still getting excess heat, pressure, and possibly oil in the tank. It also started shuddering and had very low power at the end of today and it was throwing PID 94 (Fuel Pump Delivery Pressure below normal).

Im getting pretty worn out on spending money on this engine so any help or pointers would be very helpful thanks