Clean off the crud using whatsoever works. It probably will not read the same if you turn it end for end. We are looking for coplanar, level is relative. Set the level on the ways the same way on both ends - square with bed, end of level same both ends. As said before, a level will get you close, the two collar dials it in. I may get chastised, but I have a machinist friend who sold his "precision" level because it was so "good"you just chased it around. As a tool, the machinist level is fine. Sometime we toss around tenths as if that is way off. Most of us don't have tools to measure tenths in any real sense, or tools that can actually reliability cut to those tolerances - you are looking at good grinders to get there, not lathes and mills (and not our kinds of lathes and mills).
No one buys a tool to keep it in a drawer. Yes, we all want to find the guy who did, then died and the family doesn't know that whatever it is was bought years ago for 10x what they are asking for (or has no clue). If we, as hobby guys, are whining around about use as an argument for reduced value as a negotiating tool that may be one thing, but if we really think it, we are probably full of crap. If it has to look like new, go buy it that way - but don't complain about price or quality (wih the understanding that those are probably at odds with each other). Off of soapbox, reality check statement complete.