Oil boiler smells after delivery of oil?

This is an old "Thorn Harcal 430" down firing boiler, which is working well. Problem is that there is a very strong smell of oil in the boiler lobby following a recent delivery of 200 galls of Kerosene. I have checked all the lines, unions etc. and there is no trace of a leak. This is not the first time that I have had this problem after buying oil from the same company - the other company I use mark the invoice as Domesticol 28 (sec?) I am wondering if perhaps I getting a heavier oil when I buy Kerosene from this company or perhaps the sudden increase in the head of oil is causing a problem for the elderly oil pump? Any thoughts will be appreciated. Thanks, Harry.