I would expect the gasoline market to be more competitive than heating. Transporting heating oil into homes requires large infrastructural investment and high fixed costs, so you'd expect there to be more market power concentrated in a fewer number of firms, driving the price of it up. Creating gasoline for sure requires a lot of investment too, but not for nearly a large area I would imagine.
So basically, consider heating oil an input for a "utility" sort of company, which acts as a natural oligopolist/monopolist. I'm not exactly sure what the market looks like here in the United States, but this is my best guess. Someone else can probably elaborate on this answer or provide an alternative suggestion.