I appologise for coming in late with my additions but what the. . . .
As a kid I recall the instructions that came with all my ED marine engines stated that a diesel is being cooled the right amount if is running so you can place your finger tips on the cylinder head and keep them there for justa few seconds.
I know that's a bit impirical but that's what it said.
I know that if you can't keep you finger on something for more than a second or so it's at 140 deg F. This is typical for most people. 140F for a head temp would be pretty cold for good efficiency.
Andy, how does the motor oil blend smell when running in the engine? The castor mix smells nice.