The original NACA references specifically say *NOT* to use NACA-style
entrances for heat exchangers (oil coolers, radiators).

Do they say why?

Ditto. It seems to me that the air would not care what it's function was at
the time of entrance. Since there have been a few homebuilts that used it for
just this purpose - and seem to have worked - I am/was planning to do the
same.............