Add 131.5 to the API gravity. The formula for API gravity is API = (141.5/SG) -131.5 where SG is the specific gravity of the petroleum liquid being measured.
Divide 141.5 by (131.5 + API gravity) to obtain the specific gravity of the oil.
Multiply the specific gravity of the oil by the density of water to obtain the density of the oil. This follows from the formula of specific gravity where SG = density(oil)/density(water).