i would stay away from petrolium based oils, or solvents, can have a bad reaction to sulphides,and make them deteriate, i am with red on this one ,clean them well, then clear coat them with a laquer , laqs are nasty chemicly ,but when they dry they lose their nastiness,and become stable and wont further deteriorate your specimen, oils tend to keep there penitrating power and will absorb intosome rocks,sulphides are are absorbant do to their own chemical nature,,one time i had a piece ,rather large,that iused a solvent cleaner on, and was happy with the shine ,and transluce, but left it sit like that for weeks before i touched it again, and when i did it crumbled, i figured the solvent penitrated into the micro fractures ,and allowed my pretty rock to crumble into a bunch of tiny pretty rocks. just shareing my experience.