The port light seals (gasket) on my sailboat were becoming dried out and would stick when I opened the window. I do not have any leaks (that I know of) though. I had read that the best way to freshen/recondition these seals is to work some white Vaseline into them. This past week, I took on that project.
I simply smeared on the Vaseline and then working it around with my finger and then wiped off the excess. Pretty simple process. I was surprised by the results. It really does bring the seals back to life!