It is a brass crush washer for the oil pan drain plug. It is supposed to protect the threads on the aluminum oil pan. The o-ring rubber gaskets (3) come with the oil filter, which now costs more than $20.
Mobil 1 in Northern California is usually $4.99 a quart at car part stores and $4.89 at Walmart. I usually wait for a sale at Kragen and get it for $3.99 a quart. I have seen the five quart bottle for the 15-50W but not for the 0-40W.