As the most expert expert ever with more expertise than all the other experts combined in the world history of burning oil for winter heat .... it seems that this too thick blend of bio#2 heat'n oil is absolutely, positively, absolutely, positively, absolutely the problemo here!
For a late model oil boiler/furnace to run good, everything ..... the very high 150-psi pump pressure, the fan driven air volume, the tiny tiny hole in the fuel nozzle which atomizes and sprays the fuel oil, and the fast paced electric zap that ignites the atomized oil to make a continuous hot, pulsing flame need to all be in synch for it to go good.
Is like the oil pump pressure, oil viscosity, tiny miniscule hole in spray nozzle, fan air, and electric spark are four different partners that got to work together to make it happen ..... budda, budda, budda, budda, all together now
...... to make the hot, hot, hot, sputtering flame to make the water hot
, or air hot for either the oil fired boiler or furnace.
Could be a different nozzle with a not so tiny spray hole would make the thicker fuel go good, and is not a big deal to switch out the nozzle?