What is your process liquid?
It it sheer sensitive - sheer thinning / sheer thickening?
Are there solids or particulates?
Does it solidify or deposit out in the lines?
Are you using VFD
to control speed?
If so, how is speed controlled?
May I assume you are using / want to use the same pump for the entire range of viscosity?
A gear pump (or virtually any other positive displacement pump such as screw or lobe) typically have internal tolerances are adjusted for viscosity. For thin fluids the tolerances are tight to limit internal slip (recirculation) within the pump. These tolerances increase with viscosity. A pump sized for a thin fluid will bind with more viscous fluid. It is suspected your pump was selected for the high end of your stated viscosity. If so, that pump used with a thinner fluid will have greater internal fluid slip and flow will decline. So you should expect the highest flow with the highest viscosity and lowest flow with thinnest viscosity. System friction losses also change with viscosity, increasing with viscosity. So at a constant flow the pressure will change - increasing with viscosity
What is your objective, constant flow with variable viscosity using one pump for all fluids / viscosities?