<<Like Moondog, I have just one small (5 vane) oil cooler. It's mounted on
the firewall. I built a NACA scoop on the left side of the cowling by
enlarging the drawing of the cockpit vent in the manual by 2X. Behind the
NACA scoop is a plenum that gradually enlarges to slow the air and build
pressure>>

Looks like you've done the right things.  Just as an observation, I have
seen lots of cooling attempts, including those on (most?) certified aircraft
that seem to have ignored the air flow half of the equation.  You have to
have uniform air flow across the face of the cooler to get maximum cooling
effect.  That takes, as described above, a diverging duct in front of the
cooler - or excessive cooling drag will be the result.  Three choices -
inadequate cooling, excessive drag, or a good divergent inlet.  Along with
an efficient inlet scoop in a good air flow location, of course.

Gary Casey