What you are using is fine. It is soft aluminum that conforms to the seat and plug. They can also be dead-soft copper.
OR you could use a Stat-O-Seal type washer that looks like this below. I like these.
What you are using is fine. It is soft aluminum that conforms to the seat and plug. They can also be dead-soft copper.
OR you could use a Stat-O-Seal type washer that looks like this below. I like these.