Oil immersion is recommended for making seperation negatives for the dye transfer process. This carrier allows you to sandwich your transparency and contrast and highight masks keeping them extremely flat. You can use mineral oil, silicon oil or castor oil because they all have similar refraction indexes to the glass in the carrier. As Claire has stated Bob Pace recomends the Dow Corning #200 as probably the best for the oil immersion process. According to Bob the Dow oil does not soften the emulsion like the other oils which makes damage to the slide or negative easier. The glass in the carriers must be as clean as possible and you will need some sort of thin knife to pry the glass apart after it has been in contact with the oil. It can be a messy process but the results will be worth the effort.