In several of the posts have touched upon the salient points.

The original Diesel engine war designed and developed to run on peanut oil.

Yes the newer electronically controlled engines have tighter tolerances and are less forgiving of using straight vegetable oil.

There are 3 primary reasons for processing vegetable oils into bio-diesel.

First the process removes glycerin and other impurities above and beyond simple filtering. This makes it cleaner burning and more efficient. Since it is a relatively simple process. It is worth while to do. The only real problem is the initial expense of the equipment. But a good scrounge could easily over come much of these problems.

The second is viscosity. The process does thin out vegetable oil to more the consistency of commercial diesel.

The third is temperature tolerance. Diesel as well as vegetable oils have a nasty habit of getting thicker as the temperature drops. If it gets cold enough even commercial diesel gels to the point you can't pump it from the tank. That is why there are winter formulations for diesel fuel. It is also one reason many truckers leave their engines idling when stopped for sort periods of time. Such as rest stops and meals. The Bio-diesel process lowers this critical temperature somewhat for vegetable oils.

The reasons for preheating and the duel tank systems is to help over come this viscosity problem.

It is somewhat easier to start a more modern engine on diesel than vegetable oil. It simply takes more cranking to get the engine running on straight veggie oil. But it is do able.