This could be the perfect vehicle to finally put the "do oils actually clean" debate to rest. I'd say run the house favorite, with regards to best so called cleaning abilities, about 3-5 oil changes and then pop the valve cover off and see if anything changes. I'd love to see the outcome and honestly wish I had an old sludgy car to test it on myself. Now to answer your question if you can get it try using Amsoil's engine flush, it should be gentle enough to not break off chunks of stuff but still remove it