Assuming that that's a diesel engine car, it sounds like you can get away with mixing some sunflower oil into diesel for a short time, though it'll create problems.
This talks about issues that came up in using a 50% blend of sunflower oil in diesel for two months.
https://library.ndsu.edu/server/api/core/bitstreams/1b185dd3-8962-4ce6-a192-2b9c8a206e4e/content
Long Term Performance of a Sunflower Oil/Diesel Fuel Blend