I've seen an issue on the front main seal (the one at the back of the car) where the seal wears a groove in the pulley. The fix is a new pulley, sleeve the old pulley, or (this is what we did) turn down the original pully microscopically on a lathe to clean it up. I suppose the same thing could happen at the other (flywheel) end, but as you note it's the crankshaft itself that would be the problem. I find it hard to believe that the crank would be warped enough to clear the seal and not cause a tremendous vibration in the flywheel. For the cost of fixing that you can buy a lot of cardboard for the garage floor...... OR - it's just a flaky mechanic. There are actually two places that it seals, the rubber seal itself AND the retainer that holds the seal to the block. If you really want to make a go at it, get him to open it up again, remove the seal and retainer, and either show you the crank warpage (with a dial indicator) or the groove (not likely). I'll bet its not either and he just didn't get it buttoned up right. Or you have a leak from somewhere else. I would not use oil additives on anything. Run away run away!. Dave [duplicate quote snipped buy moderator]