There is a thread on oleomargarine at http://goscf.com/t/6379&whichpage=2
You'll notice yours is for uncolored. There was an equivalent stamp -- in color, aptly enough -- for colored oleomargarine.
I am sure there are fervent collectors of margarine items out there, although most of us old-timers prefer butter, it appears.
As an aside, I recall reading a book long ago about submarine warfare in the Pacific during WWII in which, after returning to Pearl Harbor after a mission, some of the crew repaired asap to a local establishment and enjoyed -- wait for it.......
.......a stick of real butter between two slices of white bread, so disgusted had they been with the taste of their undersea oleo!