Very strongly of kerosene.
Despite being warned not to do it, while I was away erindoors let the oil tank run dry. I got it refilled this morning but the fuel line and pump were airlocked. I managed to get oil as far as the check valve inside the boiler by slackening off the nut and bleeding out the air (oil pressure in the tank was enough to push the air through). Then I had a frustrating time getting the boiler to light.
Eventually I resorted to the manual and it mentions venting the pump during commissioning using a combined pressure gauge and vent valve. I don't have one of those so I opened up the case, removed the plastic cover over the Rielo burner and examined the pump. This had a long brass thingy with a rubber grommet which I guess is the attachment for the pressure gauge, and below it a blanking cap. I reasoned that the cap was for the return fuel line on a twin line system (ours is single) and slacked off the nut and got some air and fuel foam out of the pump. Several repeats later (and piles of oil-soaked kitchen towel) the burner agreed to light and things seem to have been OK since then.
Did I do the right thing? Anything I may have missed?