When I engrave wood, I generally cover it first with release paper that one would use for lifting and placing vinyl letters (cheap and works great). Be sure to smooth it down. Well with a squeegee because air bubbles will keep your wood from staying clean. Sometimes I’ll use the orange film from PhotoBrasive, or the red film from Rayzist Photomask for the same purpose. Both of these take paint better if you were going to do so.
As for cleaning, alcohol is my general go-to in my workshop, but not for wood. For wood, if it is untreated, you can usually clean it pretty well with water or hydrogen peroxide, but it must be done within 15 minutes of engraving the wood. I’ve never heard of the oil trick, but I will say that if you were going to varnish or treat the wood with something that dries, these finishes help tremendously with the clean up afterwards. Of course, a light sanding may also work, but you don’t want to let those resins soak into the wood too long.
