Weird Harold wrote: I doubt anyone would design a Diesel engine to use the La Brea Tar Pits as fuel although the pits are technically "crude oil" deposits.
The remainders from fractional distillation are often very thick and viscous, barely liquid. IIRC, ships that use "Bunker Fuel" use steam heat to make the fuel pumpable. Again, just an engineering challenge to design an engine that will work on the fuel available -- as long as the fuel is technically liquid or can be liquefied.
Note here that "Doxford Engines" at Sunderland did eventually get one of their "76J4" turbo-scavenged 2-stroke, double acting engines to run on a mixture including 95% powdered coal, which had been one of Dr. Diesel's original objectives (IIRC the mix was 95% Coal, 3% Sea Water and 2% Heavy Fuel Oil)
Having achieved this during a pseudo-Oil Crisis they were then "Shafted" by British Shipbuilders, closed down with two years orders on the books and their patents sold off to the competition (Sulzer)