Brian is correct.  The Skelly Oil Co. had a substantial fleet of tank cars in the 1920s (more than 1,000 cars) but, like many private tank car owners, they sold them in 1933 to Union Tank Line owing to the economic effects of the great depression and thereafter leased tank cars from UTL.  After World War II, some UTL ICC-105-400 insulated high pressure cars that were on long-term assignment to Skelly for LPG service bore “Skelgas” trade marks, but UTL never applied that stenciling to conventional ARA III/ICC-103 tank cars.  Modelers may have been confused in the past by the fact that Champ offered a decal set for a UTL car with “Skelgas” logo but, though the car number was correct for a high pressure car, the application drawing showed a conventional ICC-103.

Ironically, the one photo I know of a Skelgas car showed it with small stenciling on the tank reading “Leased to Tidewater Associated Oil Co.”  Presumably that was a short term lease, or Tidewater Associated would have complained about using a car lettered for another oil company.