Hi
After several years of mixing new veg oil with diesel at no more than about 30% when I can find it cheap enough to make it worth while in my 300tdi, I was intrigued to read that some people are mixing used engine oil with their diesel - so as I had a fair bit of used oil and a lot of odds and sods of unused oil hanging around so I thought I would give it a go. I filtered the used oil (probably not well enough) mixed it with some unused oil and mixed about 15 litres to a tank of diesel just to be cautious. The engine seemed to run smoother but I noticed quite a bit of black smoke on start up and under acceleration. On the next tankful I did the same, but the smoking became much worse, especially at idle. I have since ran the tank low twice, filled with clean diesel and changed the fuel filter, but the smoking doesn't seem to be getting any better. I am now wishing I hadn't tried experimenting with used engine oil. Is it possible that I could have damaged the injector pump, or could it be some engine oil hanging around in the tank - any ideas ?

it will clear eventually, if you fill it to the brim you will see that its still black or certainly very dirty diesel you are running

Drain the tank (when almost empty), wash out with white spirits, drain this out, and then fill back up with diesel - or just wait.