If I recall and I am trying to recall what I'd read some thirty odd years ago, the vehicle that was running on the fish oil was out in the far East.Yes, anything up to the 2.5 NA diesel should be able to run on straight veg oil, though I would be careful with using anything cooked and make sure it's perfectly filtered. Might be tricky in winter, but there are additional systems like fuel and tank preheaters, starting tanks with normal winter diesel etc.
But... why waste that lovely oil? Mash everything in the can with its oil, and eat it, drinking every last drop of it. So many wasted calories by draining it away! We always scoop every last drop on hikes, giving you around 500-600kcal per tin, 3-4 tins doing you in a day if in a pinch.
But as regards the eating of the fish nope I don't discard the oils the fish are tinned with through understanding what's in the fish flesh is also now in the oil. Same with tinned veggies the liquid the veggies are tinned with are infused with what's in the veggies for it to be foolish to waste that material