[jaggederest]

Considering agriculture and forestry combined produce about 5 billion tons of methane emissions a year, I don't think there's a shortage of availability.

It'll be a lot more work to capture and direct all that, but the biosphere spins off plenty of decomposition byproducts.

[defrost]

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.