Yep. A basic diesel engine is a pretty amazing thing that, like a jet engine, can burn pretty much anything and is simple to maintain. Once you start adding all

Yep. A basic diesel engine is a pretty amazing thing that, like a jet engine, can burn pretty much anything and is simple to maintain. Once you start adding all the modern gubbins in it to make it efficient and powerful, you lose that generality. The main issue with using alternative fuels in modern diesels is the injectors: unit injectors and common rail systems are high pressure, precision components that are very sensitive to fuel quality. Used frying oil is not clean. So your mate's BMW either had some modifications to let it use vegetable oil reliably, or it's really old!