Assuming it's a gear bed oil bath mower I don't see any problems with the oil. When you get it out and lay it flat the oil will move to the ends of the bar. You might want to lay it flat, turn it over a few times to get the oil circulating, then let it sit for aminute or so to let the oil settle into the bearings, but since the gears act as little oil pumps I cant's see any problems. I would be more concerned with safely chaining the bar upright to a post or something so it will stay upright. I've only seen one mower that was designed to sit upright when taken off the tractor.