I veto the magmeter idea.
Magmeters are for conductive fluids only (water or water based solutions, acids, bases) and hydraulic oil is not a likely prospect for a mag. Unless hydraulic fluid has some conductive ions floating around in it (salt?), it's not a suitable medium for a magmeter.
The killer app flow meter for hydraulic fluid is a turbine meter: accurate, high turndown, and the oil provides lubrication for the turbine bearings. Check out
http://www.hofferflow.com/
Kobold has some inexpensive paddle wheels, the DF series, (not to be confused with a turbine meter), but be wary of the pressure drops encountered. The DFs have high pressure models.
I suspect that coriolis (mass flow & expensive) will have a problem with the pressure at 2000 psi (the wall thickness of the coriolis tube has to be lightweight and thin).
If the temperature of the fluid is fairly constant and not hot, you might consider thermal dispersion. Supposedly they all claim to be temp compensated, but . . .
People like Thermal Instrument or IFM or Kobold. Again pressue is could be an issue.
Kobold makes a thermal dispersion insertion type, the KAL, but it is calibrated for water flow, and I'm not sure how you'd convert to oil flow (all media have their own thermal dispersion rate). But I've found thermal dispersion works best on cool liquids and overall success drops as the temperature rises.
Vortex shedders might work, depending on pressure rating.
Differential pressure can be used at those pressures, with a 3:1 to 5:1 turndown, meaning flows below 4-5 gpm get noisy.
Dan