Verifying correct formula for pumping oil

[Mach3Maelstrom]

I have a motor-pump that's pumping oil through a plumbing system. I want to choose the correct electrical motor size for this application.

Which equation should I use to determine the size of the motor I should purchase (in horsepower)?:

HP = (GPM * PSI * K) / (n * 1714)
where K = 1 for electric motors and n = efficiency of system

OR

W = Qpgh/n, then convert from W to HP
where W = power (in watts), Q = volumetric flow (in m^3/s), h = head fluid has to be lifted (m), and n = efficiency of system.

I'm an electrical engineer, so I'm a bit rusty with what-mechanical-equation-should-apply-where.

I was advised the first equation by a mechanical engineer, but I keep getting the feeling that's an equation for pumping water, not oil (which is more viscous), since the equation doesn't use any variables regarding the fluid it's pumping.

Thanks!