First off, sorry for the long post.

Ok, I'm new to the Audi world so I've got a few questions on some maintenance of an 2005 Audi A4 1.8L. I've been reading about oil and weights and from what I've read, M1 0w-40 or Castrol 5w-40 (meeting the VW 501 specs). What about a filter? I would like to use either an M1 or Wix filter but read something about a flow back check valve. Do either of these have this valve? What other brands have this valve? How much does a filter from the dealer run?

Also, I read that you have to replace a crush washer/lock washer on the drain plug? What is this and is this something that you can get at Autozone, or is this dealer specific item?

Last question, about Seafoam. If it does trip a check engine light, it this something they can clear at Autozone, etc or do they have to do it at the dealer?

So, am I good to go with either one of those oils and filters? What about the crush washer?

Thanks in advance for the help guys.