Hamlin's Wizard Oil poster from 1890
While not literally snake oil, Hamlin's Wizard Oil was sold as a miracle cure for just about everything, as you can see from the list on the poster above. There is little doubt that the concoction cured anything. Despite its lack of efficacy, it was marketed aggressively and sold many units, such that Hamlin's elected to print cancels rather than only use handstamp or manuscript cancelling.
The following two images are from the Joyce/Chappell list of battleship proprietary printed cancels, published in the 1950s. In this list, Joyce describes the types of cancels that can be found, however, in the years since the production of this list, many dates have come to light not catalogued by Joyce. In Henry Tolman's collection below you see may of those missing dates. There must be more out there. Do you have any?