If your engines getting smokey first try driving it very hard when warm, this usually clears out a diesel (is normally carbon build of from pottering around at low revs), if that doesn't help then try FORTE diesel cleaner, best on the market. Personally I would never use a petrol mix on any modern diesel, an old 200 or something then yeah not so bad but anything with more modern systems can suffer from deal degradation. My 406 diesel has lost some MPG thanks to a customer filling it up with diesel, I suspect injector seals are damaged but its still running and not worth repairing so I won't bother, had it been run a bit longer I suspect it would be dead now...