The density is given in mass per unit volume (g/mL) in this case.
Let ρ = density (in g/mL)
Let V = volume (in mL)
Let m = mass (in g)
ρ = m/V
To find how much mass (how many grams) of oil is needed, solve for 'm'. Then substitute the values in for ρ and V. Make sure that the units cancel out put leave the mass in grams in the numerator..