Excavations at the city of Samaria carried out in 1910 uncovered ostraca in the storerooms close to the king's palace (8th century BCE), and of these, ten bore an inscription reading: 'nevel šmn rhs'. Many studies have dealt with the term 'rhs' and the consensus seems to be that it refers to a type of oil produced from the olive tree. Our study shows that the term 'šmn rhs' which appears on the Samarian ostraca was preserved in several medieval sources which relate either to the choice olive oil that was produced in al-Sham (the Land of Israel and Syria) under its Arabic name 'Zait al-maghsūl', which means 'washed oil', or to the oil of Palestine known as 'Zait al-Filastini'. The oil underwent a process of washing in water to remove its characteristic accompanying taste and odour in order to transform it into a basic, neutral oil, probably for the purpose of mixing with spices or other aromatic oils. Evidence of the technique of oil production and the vessels referred to in the Arabic sources may be found at several archaeological sites in the Land of Israel which are dated to the Iron Age. We suggest that the oil in question is a distinctive variety of oil, one of the exclusive quality products that were stored in the 'treasure house' of the kings of Israel.