Ant,
This system is designed so that it will draw off 5% to 10% of the oil pump output, depending on oil viscosity, oil pressure, etc.
There is generally a minimal effect on oil pressure, perhaps 3-5 psi at high rpms. One thing that works in your favor is that you are increasing the effective sump capacity by 1.5 to 2.0 qts in the case of a pickup diesel engine. This tends to reduce equilibrium oil temps and improve oil pressure.
The other thing I like to do is mount the unit in an area of high airflow, so that it functions as a supplimental oil cooler.