When you apply oil to a shell, some of the oil actually penetrates into the
substance of the shell, and some of it remains on the surface of the shell.
Getting oil into the crystalline structure of the shell is the objective.
That's why some people dilute the oil with a solvent - to make it less
viscous, so it can penetrate more easily.  Once oil has filled the porous
spaces in the microstructure of the shell, you want to remove the residual
oil that remains on the shell surface.  Otherwise, the shell looks and feels
oily, leaves oil spots on your labels and cabinet drawers, and attracts
air-borne dust like a magnet.  With a smooth shell like a Conus or Cassis,
oil can be applied with your fingertip, or a cloth or brush, and all the
residual oil can be removed simply by wiping with a clean, dry, absorbent
cloth (I prefer old bath towels).  But with a heavily sculptured shell like
a spiny Murex or Spondylus, or a rough-textured shell like a Morum or
Morula, you need a small brush to get the oil down into the pits and
crevices of the shell.  And once the specimen is oiled, you cannot wipe off
all the residual oil.  For such shells, I use warm water, in a fine, high
pressure spray (one of those extension hoses that come on many kitchen
sinks) to remove excess oil.  If you use undiluted mineral oil, you should
leave it on long enough to let it penetrate, before removing the excess oil
from the surface.  Probably 15 minutes or so is enough.  If you use diluted
oil, you should allow the solvent to evaporate before removing the excess
oil.  15 minutes is usually enough for that too - most organic solvents are
pretty volatile.  In using such solvents, don't neglect health
considerations.  Remember, when the solvent evaporates, it doesn't
"disappear" - it mixes into the surrounding air, which means you will be
breathing the solvent if you evaporate it indoors.  Also, some solvents can
be absorbed through the skin, so oil diluted with organic solvents should be
handled with care.  Obviously, the level of potential danger from either
absorption or inhalation depends on the particular solvent used, and the
amount of exposure.  Something like benzene or chloroform or gasoline is
highly hazardous, while xylene or kerosene or mineral spirits is RELATIVELY
safe.  Also, keep in mind that many of these solvents are moderately to
highly flammable, even though the mineral oil itself isn't.
Paul M.