Either the finest oil or oil- kerosene mixture can be used. Clean, pure water should also work well, and may be the best. Some people advocate scoring the metal lap. The only reason I can think of for doing so is if you intend to polish with a continuously running source of water and believe the grooves help remove the crud, or if you want to use the slurry in the polishing process and think that the grooves help hold a reservoir of slurry. If you intend to polish only with the embedded diamond, you must keep the lap surface free of crud to prevent agglomeration and rounding of facet edges.
Scored grooves can hold crud and help accumulate agglomerates.
There are also laps which are neither ceramic nor metallic, like the Last Lap. I have not used these, so will have to let others tell about them, but if diamond is used the above principles should apply to them also.