Your question was price controlling/subsidising, nobody was telling people they HAD to switch to motorcars in the mid '30's, they were all in by then. Nobody at that time were penalizing, placing restrictions on, or adding made up taxes on those horse dependent trades You cite.
And crude base/mineral oils had way more applications than just fueling competition for horses. It wasn't strictly directed at "replacing the horse". And the horse had, and still has, virtues that make it a valuable form of transportation in a number of situations. Nobody is telling ferriers to stop shoeing, trainers stop training, or restricting horsey flatulence/poo to penalize/discourage their use. Access to certain avenues is restricted, for the safety of the horse, & sanitary prudence..
