You will also be much better able to seal your coverslip to the slide. Seal the edge of the coverslip with clear nail polish and let it dry. If you have excess mounting medium around, the oil may mix with it and cause major quenching. By pulling off the excess medium, you lower the likelihood of this problem. Sealing the coverslip will also help a lot.
Immersion oil has a high surface tension.
If your specimen is not properly mounted and sealed, the oil is going to pull on the coverslip during focussing and will cause specimen translation. If it is floating around on a puddle of mounting medium which has a lower surface tension, it will be pulled right off the specimen and destroy it. Mounting Medium Mounting media can be prepared in the lab or commercially available ones can be purchased from several of the companies dealing in fluorescent probes. The pH should be adjusted to between 8.5 and 9.0. This pH has been found to be optimal by many investigators in preventing fluorescein and rhodamine quenching.