I don't think WD40 will harm most any finish. It is a pretty good cleaner and would be where I started. My next move would be lighter fluid. It leaves very little film if wiped off so it should not collect gunk. If the hinge is stiff in operation really soak it in WD40 then put a drop of penetrating oil on each hinge gap. move it through several open and closes and wipe off as much oil as you can. Oil really collects dirt so you want as little as possible, but enough to ward off corrosion. Especially if it appears to be steel.