It is my understanding that crude must be heated to boil out the water and filtered. Some modifications might nave to be made on injectors and injection pumps. This modification seems cheap and small when compared to a high pressure refrigerated system required for natural gas. The diesel part would have to be maintained for ignition of the gas in the engine or a spark system would have to be added to the engine. Then there is a whole new infrastructure to deliver the natural gas also.
The crude oil is typically already through this stage before it is sold as crude oil. On the crude oil production unit's I have been part of, we used electrostatic precipitators downstream of the filter separators.
You need to understand that nearly every crude oil stream is different containing varying amounts of Alkanes, Naphthenes, Aromatics and Asphaltics. There are other impurities like sulfur, heavy metals, etc.
There is a reason oil field equipment almost never runs directly on crude oil. It is why large offshore platforms will have their own topping units to make their own diesel. It is the lowest cost option to operate.