It really doesn't matter. That's the real answer. The extra friction loss caused by the slightly smaller diameter won't even be 1/10 of a psi because of the very short length.
Also, this is on the pressure side of the pump. If it was on the suction side you'd use the larger one.
If I could get you guys to understand Bernoulli's equation for fluid flow you could approach a problem like this with some knowledge. Basically, the equation says that any liquid has a certain amount of energy. It can be in the form of pressure, velocity or height above the datum or zero line. I can't remember if temperature is included but I think it is. The fluid can transfer one kind of energy into another and back, taking into account losses. So the oil has a certain velocity, pressure and height. In this application we can disregard height and concentrate on velocity and pressure. The oil transfers some of its pressure energy into velocity energy as it goes through the smaller diameter section of pipe, but when it gets to the other end and velocity slows down, most of that energy transfers back into pressure.
It seems that most people think of fluid flow as toothpaste squeezed out of a tube, that it stays the same diameter and if it has to go through a smaller diameter, less goes through, but this is wrong. Bernoulli's equation describes the reason why it works that way.
End of lecture.
R.
