From the late 1940's to the late 1990's, the "horsehead" pumpjack was an icon of the local oil patch.  It's up and down motion drew the oil to the surface from the depths below.  An observer "in the know" could tell from the speed of the pumpjack the heaviness of the oil being pumped.  Conventional crude would be characterized by a rapid stroke.  Heavy oil would sometimes mean that you had to actually stop and peer at the pumpjack closely for a few seconds to see whether it was moving or not.  The photograph below shows a "horsehead" from the 1950's.