I recommend going to a petroliana swap meet listed on the "Events" page that is linked at the top of this page. Find a vendor with a bunch of old motor oil cans and ask him to show you the fine points of dating cans by the way seams are made. Most vendors are also collectors themselves. Be courteous and respectful of their time and you may get a friendly education. Especially if you are also a customer.
Lots of helpful info is in past years of posts on this forum, found by using the Google Custom internal search form at the top of this page or an Oldgas forum specific search by clicking "Search" on the menu near the top.
Find threads such as this:https://oldgas.com/forum/ubbthreads.php?ubb=showflat&Number=490517