Really depends on the engine. If you have common rail injection, then it's a big nono. The viscosity of the oil is much, much greater than diesel and will quickly ruin things.
My truck has an engine that's largely unchanged from the 1960s design and it really doesn't care what you put in it (within reason). Cold weather can be an issue as the viscosity increases with temperature, but I've heard of people near Aberdeen running 50/50 mixes all year round without issue. Others go for a two tank setup, running on our diesel until engine up to temp then switching to pure veg.
Mot testers like you to have pure diesel in for the Mot, as otherwise the test station ends up smelling like chips for weeks.
You lot have that advantage over there as we have fewer diesels than we should. I especially like the Rover TDis and we are (the NAS market) finally getting one next year!!
Did you ever see such a messed up situation in your whole life, son?
Nope. When your engine is hot it's fine (viscosity is a function of temperature) and a two tank system uses pure diesel to get up to temperature. Otherwise mix it in the tank with diesel and it's thin enough.
Of course I'd never do that in practice, as that would be tax evasion. Even if veg oil is half the price of diesel, I'd never consider it, Officer.
