Abstract: Base oil can be recovered from contaminated O/SBF by combining a chemical process with a mechanical process. The chemical treatment includes adding a demulsifier, an anionic surfactant, a non-ionic surfactant and/or a mutual solvent to the contaminated O/SBF in an amount effective to separate the base oil from the contaminated O/SBF fluid followed by mechanical separation of oil from water, and optionally from any solids present. The recovered base oil (i.e. conventional drilling fluid, conductive drilling fluid and constant rheology drilling fluid, etc.) may then be reformulated to make a new OBM of the same type from which the base oil was recovered, or as a fuel for engines.