I use a UV filter on my lens, but have had problems cleaning it.
It appears to have greasy film, both inside and outside, and has been like this since new.
I have tried cleaning with a very mild detergent and water, also using a micro cloth for spectacle cleaning, but all to no avail.
All I seem to do is change the smear pattern!
I can't say I have ever noticed that it has affected my images, but would like to have it "clean".