Which oil

I have a 318 in my Demon that has a tired bottom end. Lacks oil pressure when it is warmed up at an idle. Holds 25 lbs at the least running down the highway. I am in the process of building a 440 for it but am not pulling the 318 until it blows or the 440 is ready. Right now I run 10-40 Valvoline in it. Has anyone ran diesel oil in a gas motor? Like Rotella 15-40 or other brands? Just trying to extend the life of it so I can keep driving the car till motor is built.