Aside from the fact that the Military are far more interested in reliability and ruggedness than fuel economy, what is the fuel efficiency of the multi-fuel engine, I believe I’d be finding that out first, then comparing the costs second. You may find to your dismay, that the multi-fuel is a gas hog, and yes it does make a difference, even when using “Free” WVO, there are still the costs of storage, filtering, additives, (Prevent gumming and freezing when very cold weather) and the time and labor involved in getting, transporting, and storing the oil. Also don’t forget some sort of a backup system, either a second source of oil when/if the supply “Dries up” or a backup , say electric heat , if the engine fails, or needs repair. We had a very interesting system sold by our local Holliday Inn, they had a “Caribbean” system that used three very large Catipillar V-8 diesels converted to natural gas (Lowered compression, spark ignition fitted) and used for the total power requirements of the Hotel, engines ran generators to provide electricity, hot water, and used a huge absorption air-conditioning system, also run by the hot water (They even had huge water jackets on the exhaust to catch as much heat as possible.) run one at a time, with a backup (The third,smaller engine) as an emergency standby. This all came to my attention about ten years ago when they ran an article to sell the units, as the cost of natural gas had sharply increased, making the system no longer economical to run, they had been beating the Alabama Power rates by better than half the cost, and had the Hotel designated an emergency command post under the Civil Defense act, (Paid part of the cost to ensure a powered Command center in case of National Disaster (Unnamed)), but the sharp increase made them switch to High Line power.
I know that fuels are only part of the issue here but a used multifuel diesel engine from an army 2.5 ton truck can be bought for $3500. In fact that price buys the entire truck. My neighbor just bought one and I am very envious. 3 axles and they all pull, I think. Anyway, I think the army has backed off some on the claim that this spark assisted diesel can burn any fuel but even still, it is a heck of an engine. I would love to have one on a metal rack to use for stationary power for electricity and other uses.