What function do high TBN levels in an engine oil play in gasoline fueled engines? Many posters quote using a HDMO in their cars because of the high TBN levels. I always thought that a high TBN was necessary to combat acids formed from the use of high sulphur fuels, and not so much a concern in other applications. Are people buying oils based (no pun) on the TBN and zink levels and do oil companies ut excessive amounts of both in an oil to sell the product? What roles do boundry lubricant additives play in a gasoline engine that has a low drag rollerized valve train and a 2:1 con-rod ratio that eliminates piston side loading?