Sodium can be used as a detergent/dispersant additive . If the lab don't look for it in a VOA it would be difficult to tell how much if any was initially used .
Here are some other odd additives . Lead , copper and barium . Oh the future is bright . Exiting times for new engine oil formulas
Barium (Ba) Detergent or dispersant additive
Boron (B) Extreme-pressure additive
Calcium (Ca) Detergent or dispersant additive
Copper (Cu) Anti-wear additive
Lead (Pb) Anti-wear additive
Magnesium (Mg) Detergent or dispersant additive
Molybdenum (Mo) Friction modifier
Phosphorus (P) Corrosion inhibitor, anit-wear additive
Silicon (Si) Anti-foaming additive
Sodium (Na) Detergent or dispersant additive
Zinc (Zn) Anti-wear or anti-oxidant additive