So how do they measure up on rarity (ie because they are so breakable) and did it also make pre WWII metal cans rare because of all the metal drives?
Oil jars are not uncommon, some companies may be though. The hard part is finding one with the label on it. Most of the time after use they were repurposed into something else. Rarity depends on who wants it, how many made, etc.
Me being a Mobiloil collector think the paper WWII versions are much tougher to find. I have had numerous Mobiloil jars and only one paper WWII version.
I am sure this varies based on manufacturer as well.
As for Pre WWII cans being more scarce, i can see that they may have used the old scrap to make new metal things. So yes i believe that plays a huge role in not being able to find some old cans.
Here is a pic of a paper one like i was talking about.