Oil immersion for microscope objectives is a different
thing from wet gate printing. The oil used has an index of
refraction higher than air so the angle of incidence at the
entrance pupil of the objectective can be larger. An actual
oil is used. Wet gate printing uses an alcohol. The index of
refraction is chosen to be near that of the support. So, the
liquid fills in the scratches on the support with a material
which does not diffract light at the scratches. There have
been several solvents used in the past, most of which are
now considered environmental hazards. 1,1,1,trichlorethane
and perchlorethane were common for this application as well
as film cleaning. Now very dry Isopropyl alcohol is used for
both cleaning and wet gate printing.
 Because the liquid matches the support its most effective
on scratches on the back of the film. It has little benefit
for scratches in the emulsion. Emulsion scratches tend to
disrupt the image anyway so even a liquid matching the index
of the gelatin would be of limited benefit.