I'd guess that using oil on that large stone wouldn't ruin it. It appears non-porous, and a good cleaning may be all it wants. I'd squirt dishwashing soap on it and get a 2$ china double grit synthetic alumina oxide rock, coarse side first, and scrub it until my arm got tired. that's going to get much of the surface lapped, the finer scratches at least, then swap to the fine side of the china rock and repeat.

Scrub off with a green pad, or a stiff brush, to remove any abrasive bits, and give it a try on a big knife. That will also smooth it up some. I think it's a butchers old school finishing stone strictly by the size, and using it for a razor may be possible but i'd doubt it was the original purpose.