Diesel fuel
In a modern diesel engine, the fuel is part of the engine lubrication process. Diesel fuel naturally contains compounds that provide lubricity, but because of regulations in many countries (such as the US and the EU countries), sulphur must be removed from the fuel before it can be sold. The hydrotreatment of diesel fuel to remove sulphur also removes the compounds that provide lubricity. Reformulated diesel fuel that does not have biodiesel added has a lower lubricity and requires "lubricity improving additives" to prevent excessive engine wear.