For immersion oils with a range of refractive indices, we use Cargille's
Laser Liquid, Series 5610.  These are very low fluorescence, non-toxic,
UV transparent, safe for the lenses (some oils corrode the mounting
adhesives-- not a trivial issue, as it turns out), and you can get them
in a range of refractive indices (1.510 - 1.5300, I think).  Any oils
within this series can be mixed to generate a desired index of
refraction.
 
As for oil leaking down lenses, we assume that the users will be careful and
seal the lens with slicon based adhesive.  This removes the use of the
spring mechanism, but saves the lens.  In general, you then should teach
all users the "art" of blind focusing using the field diaphragm.
 
By the way, one Nikon rep told us that we should use their oil since
it was a low dispersion oil.  Succinctly put, this is a load of crap.
All oils will have measurable dispersion: their refractive index will
change as a function of the wavelength of light.  This is why for
optimal imaging, especially in confocal microscopy, you should find the
correct immersion oil for a given specimen.
 
Jason Swedlow