Has gas, fire, no start

When the car stopped running it sounded like it was starving for gas or it was only running on two cylinders. I found oil inside the tubes that fit onto the spark plugs. So I replaced the plugs, plug wires and the valve cover gasket. That didn't work. So I replaced the fuel filter and checked to make sure that the gas is coming out of it. Still won't start. So I traced the fire from the coil pack to the plugs, and to the fuel injectors and all is there. Hope you can give me some ideals about what it could be. Thank you