Steve,

It is difficult to specify a delta P that would be suitable for effective cooling because of all the other variables in the cooling system.  In my case, 2 inches of water pressure across the core of a Fluidyne DB-30618 oil cooler is more than sufficient to cool the oil at 85 deg F OAT in climb or slow flight.

Average pressure at the exchanger face with a piccolo tube is easier to measure and interpret and gives useful information concerning the behavior of the duct.

Velocity measurement requires both a pitot and a static pressure measurement at the location of interest.  This combined with the area of the duct and the air temperature at the location of interest enables the calculation of mass air flow rate.  Combining that with the delta T of the air across the exchanger core enables the calculation of the rate of heat exchanged.  This can be compared to the rate of heat exchange required as estimated from the fuel flow rate and engine characteristics and serves as a reality check.

When assisting builders of a Zenith 601 with a Jabiru 6 cyl engine installation with cooling system concerns, we used four pressure sensors of the type you have except that they were differential pressure sensors.  We used the 5 volt excitation from the Dynon D180 to power the sensors.   We used the two normal fuel level inputs along with two of the GP inputs all configured for capacitive fuel senders to log the data.  An additional logging instrument was not required and no additional pilot workload was introduced when collecting data in this way.