Never get oil on the outside of the bottles. Good lab technique would be no oil on the outside of the bottle. If you get it there, clean it. Don't leave oil for the next person.
Rule: oil bottles may never be put down anywhere except in a petri dish.
If you need to move the bottle, the dish under it moves with it.
Why are there so many bottles of oil?
Immersion media (which are not always oil) are a lens element between the microscope objective and the sample (just as a specific coverglass is a necessity). Each one has a specific index of refraction.
It is essential to use the correct immersion medium and to not contaminate any of them.