Probably not sludge then. There's a baffle in the oil filler hole that makes it difficult to tell if there's sludge in there, but you can try looking. Best way to check is to remove the valve cover.
As far as the cost of replacing the valve seals, I'm not sure because I've never had to do it. Maybe someone else can comment on that.
If it's just the seals, then it's not too serious. It can be driven quite a while, without damage. But, over time it will get worse. Do you have to add much oil between changes?
Engine cleaner (detergent) in the oil can help. But, I'd still check for sludge before I did it just to be sure. Putting detergent in the oil of one of these engines that happens to have sludge will clean some of the sludge out, but may leave clumps floating around that can clog passageways resulting in damage.