I often wonder if sometimes, we as collectors, let a few pieces of petroliana fall through the cracks, so to speak, because we think a piece is a repop or fantasy piece because the piece doesn't seem right or have never seen another one before. I think the majority of the time, we get it right but also think there are pieces out there we have never seen. Maybe some Mom & Pop or small advertising company made a few things for these gas/oil companies on a trial basis to see if they could get some market share. Its entirely possible!?!...... If I had to make an educated guess on the sign without seeing it in person, I would say it is a real sign based on the size and subject content (HD Motor Oil). It would be an ideal size for an Oilzum oil rack.
