You're right, the 20 gallons I have won't go all that far, at least on its own.

I was thinking of it more as an additive, to bump up the heat value of low-grade fuels like sawdust, wood chips, bark, junk coal, etc. Once up to heat, and with a good draft, each would help the other burn.

In that context, a cup or two of used motor oil might be very useful. It might also be highly available when other traditional fuels are very scarce.