There are a lot of studies that proves waste cooking oil, or even the Vegetable oil can be use as fuel. Some countries use this even in pure forms. However we should understand that it'll cause some problems with the engine. Cooking oil is much viscous than fuels (diesel). Meaning, it is thicker so the engine will have a hard time burning it (combustion) resulting to an unburned fuel which may clog the engine. It may be useful for now but not in the long run.

BUT, waste cooking oil as substitute for conventional fuel is POSSIBLE. A lot of studies proves its efficiency and of course it is eco-friendly and economical. We have to take note that it is better if this WASTE COOKING OIL is converted as BIODIESEL before using it directly on our vehicles. We can use a process called transesterification.

"Transesterification is the process in which fat or oil reacts with an alcohol to form esters and glycerol. A catalyst is used to improve the reaction rate and yield. Because the reaction is reversible, excess alcohol is used to shift the equilibrium to the product side."

Even McDonald's adapts this system, wherein they used their waste cooking oil as biodiesel to fuel their delivery services.

https://www.greenbiz.com/article/mcdonalds-recycle-cooking-oil-power-its-vehicles#:~:text=By%20converting%20the%20cooking%20oil

They collect this waste cooking oil from 900 restaurants in UK. This is one of the major challenge if we will apply it in the Philippines. The logistics, the source of waste cooking oil. The collection of the raw mats and facilitating it.

So to summarize, waste cooking oil can be a substitute to fuel. However, it is more effective if it is first converted to biodiesel than using pure or straight cooking oil that just undergo filtration.