Is it more or less energy dense than oil?

    The consumption of oil has steadily increased over the last three decades, totaling 4.53 billion metric tons in 2023,

That's oil shipped and used, not methane drifting free from biowaste across the plant.

How much can we actually capture, how much energy is that, and how great is the conversion challenge to shift most cars to methane, etc?

It appears a little larger than a handwave problem IRL.