if you measure oil pressure coming out of the block .. and then when it returns to the block ..... Can that tell you if there is a restriction in the oil filter ..oil thermostat ...oil cooler ... loop ? I would guess there is going to be a drop but what is acceptable ?

Mike,
I did this test on my motor, put a gauge and shut off before my cooler.. if I can remember the numbers..
I cant pull them out of my head at the moment.. but I think I saw about 10 or so pounds of drop at 2500rpm and 20w 50 Valaline oil...
It will tell you if you have a restriction.. My main issue was what was the psi entering the block.. if that is good then a slight restirction isnt a problem.. I am running a Fram HP6 filter setup with #10 oil lines..
Hope this helps..
D*ck

Thats exactly where I am coming from .. I used the Russell AN fittings with pro clasic hose and one of the last fittings I put on the hose sliced into the inner part of the hose and created a flap in the path.... I redid that end but then got to thinking about the other 10-15 fittings that were already installed .... Also #10 lines ... I used the stock merc remote oil filter mount ( drilled out all the openings) ..then KE remote thermostats then thru 3x18" merc HP coolers ... I think a permanent Pressure gauge (mech) on the return to the block will be a wise move ....

I found I had two gauges.. that read the same and were low.. I ened up mounting my electric sender on the transom and then added a Auto Meter Mech gauge tee'd off from the electric sender... So Now I can bump the hatch and take a look to verify both gauges..

installed a mech gauge at the remote adapter (return) .... About 10lb pressure drop after the Filter/thermostat/cooler loop ...Gives me a good base .. just did the port motor but I guess I wont be happy til I add the other side too ......m

I hear u..
I ran across my notes yesterday on this subject. At the output of the motor I had 80psi at 1200rpm and 85psi at 2500rpm. Then I moved the gauge to the input of the motor and saw 70 and 75psi.. So it was a 10# drop across my cooler and HP6A filter, with the 20W-50 Valvoline. I cant remember how warm the oil was, but I believe I had been running the motor for a while so it wasnt stone cold...

D*ck

Mike,
Here is a pic of how I set up my rear oil psi gauge and sender for the dash mount electric gauge...