I also think it's a real sign. The Sinclair pump plates are thin for some reason. Usually, with the fake ones the back isn't porcelain and you can see the "rack" marks from when they fired it.
Your sign is real.

WANT TO BUY VINTAGE OIL CHANGE DOOR JAMB STICKERS. ANY AMOUNT ANY COMPANY.
Buyer, collector and reseller contact me with anything you have for sale