Can use diesel to flush the engine, but easier to just use one of the over the counter products. Run the engine with the old oil to warm it up, then drain. Chuck some cheap oil in along with the flush additive and idle the engine for 20-30 mins. Don't drive it or put any load on the engine. Drain again and replace the filter and fill with good oil. You will be surprised how dirty the 30min old oil will come out if it's never been flushed before.