Years ago I used a liquid carrier on an 8x10 durst condenser enlarger to make color separations The carrier required a frame for the slide to rest in,. otherwise it would float. We would hinge the slide with a piece of tape and then carefully drop by drop fill the frame space .with the fluid . We used regular baby oil or glycerin in a pinch then drop by drop a light film of oil was placed on top of the slide and the cover glass gently lowered on the carrier. It was a messy process but at the time hide all the scratches on the base side of the slides. When we got scanners we still oil mounted them on the drums for the same reason. We never used it to hide dirt. Just careful cleaning and neutralizing any static took care of that. I have never tried it with B&W all my enlargers are equipped with cold lights negating the need to hide base line scratches.