Issues we have found with burning oils.
Most of the time your oil will come from the same source, so your heater will sit on the same delay and pump times all the time and nothing changes, however there are times when your going to receive oil from other suppliers who you don't know the purity of the oil as we did last week. The first 6, 20 litre containers where hydraulic oil excellent stuff burnt very clean, with no real ash build up in the burning pot. We had to change our settings from car engine oil we were burning from 6 second delay, to 13 seconds delay for the hydraulic oil.
However went to the next container started to pour it into the heater container, and saw it was very thin so didn't go any further. Lit the heater with that oil I poured in, and the water temperature shot up from 64-85c very quickly. This heater had been running all day clean as it should on hydraulic oil, then noticed every time the pump pumped smoke would show out the flue, not alot but enough, so I increased the delay time from 6 seconds eventually to 12 seconds no change with the puffs of smoke although the water temperature did decrease, it wasn't so much the temperature of the water but the rate of increase which said to me there's either alot of either petrol or diesel in the oil, and I think it was the latter as petrol won't create smoke. By this time the day had warmed up so I turned it off. So to get around this I poured that oil mix into the waste oil drum where the diesel will be mixed at a lesser consistency than it was in the 20 litre container, now the next 20 litre container is the same but I could smell petrol in this one, so we won't be using it as heating oil but for starting the heater from cold only. For your own safety never start a hot heater with petrol never! You'll get a meter away from the heater before it goes bang. And it isn't a small bang either, it's not so much the bang it's the skin burnt off your legs, the flue changing to a rocket, and the black sooty ash everywhere, it's a big cleanup job.