You can put vegetable oil in your car if you have a diesel engine and you're using the vegetable oil as diesel fuel to power the cars engine.
However you cannot put vegetable oil in your cars engine for engine oil as it would ruin the engine as vegetable oil does not have the same lubricating properties as normal engine oil and it will destroy your engine.
But in a diesel powered engine the vegetable oil can be run in the fuel tank to power the diesel engine.
Vegetable oil will run in a diesel engine.
Older model diesel engines run better on vegetable oil than newer diesel engines but even newer diesel engines can and will run on diesel engines but you may need to modify some diesel engines to properly run on the vegetable oil.
Although you can use straight vegetable oil to run a diesel engine it's best to use biodiesel which is vegetable oil that has been modified to improve it's performance and compatibility when used in diesel engines.
Straight vegetable oil is thicker than diesel and can congeal in cold temperatures, so you may often need heating elements in the fuel tank and fuel lines to ensure proper flow.
And some diesel engines may require some modifications to the fuel system to handle the different properties of the vegetable oil.
And straight vegetable oil can also contain impurities so a filter system is also recommended to prevent clogging of the injectors and fuel lines.
Straight vegetable oil is also more viscous than diesel and can lead to engine problems if it's not heating and filtered properly.
And starting a diesel engine on straight vegetable oil in cold temperatures can be difficult so you would be best to have a separate fuel tank for the vegetable oil and your normal diesel and use the diesel to get the engine warmed up and started and then switch to the tank with the vegetable oil.
Straight vegetable oil also has poor combustion characteristics so it can lead to carbon buildup in your diesel engine.
And straight vegetable oil can also produce more emissions than conventional diesel fuel which could cause a fail in an emission test.
While you can run straight vegetable oil in diesel engines it's not recommended for long term use unless modifications are done to the engine.
Biodiesel is a better option when you want to run vegetable oil in a diesel engine and especially for long term use.