There are two docs published by Nikon regarding the use of oils. Most people think you can use any old oil for any microscope, but this is not the case. Zeiss, Leica, Nikon and Olympus all have specific chromatic, spherical and viscosity values, which are matched to work with the corrective properties of the lenses.
Thus one should keep Leica oil on Leica lenses, Nikon oil on Nikon lenses and so forth. Below are two comprehensive documents that describe which oils to use with Nikon systems, one that specifically covers the perfect focus. I’ve clipped a revealing example of how much oil can affect the performance of your instrument. Note the massive chromatic shift in Z when using non-specific oil on a 100x lens (btw this should be “Cargille”…guess I’m not the only one with typos – heh):
I’ve uploaded the two reference docs that go into specifics to my files and info page, or you can get them here:
Perfect Focus and Immersion Oils
-Austin