Manual calls for cleaning it by soaking in mineral spirits. Many here (myself included) use a spray degreaser like Simple Green or one of the citrus brands, and rinse with water.
Once it's clean and dry, don't over-oil it.
Also, there's no performance gain with the K&N... so no rush on replacing the stocker if it's in good shape. When I need to replace mine, it'll be with a K & N, but as long as mine's in good shape, I won't be replacing it.