1 . You don´t need specify key . I have element brand about 12€ and it´s ok.
2. The best form to avoid leak it´s good o-ring. If you put PTFE it´s only a solution short-middly time.
3. Nope and it´s not neccesary. Great quality stock parts.
4. I use gas without oil silicone and it´s better...