Get a whole bag of them. It's like $0.09 each or something. You don't HAVE to change them, but why not for that price?

They're crush washers so when you tighten down on it it crushes to create a seal. Get new ones.

Oh, and ~4 quarts. Put 3 in and check to see where you are with the dip stick. Keep adding till your at the 2nd dot.