"Do you need a filter" is a perennial topic, which will be argued for decades. But you do need one sometimes, such as when I photographed an event at the beach on Saturday.
It is now covered by a film of dried seaspray with possibly embedded sand. I don't want to jump in with lens tissue, this will probably be like using sandpaper, literally.
What to use to clean it? Warm water with dishwashing liquid gently applied to the front (as the rear of the filter is perfectly clean) or remove it from the lens and then clean it?
I take it no-one would put it in the dishwasher