Volume of oil V = π*r^2*x, where x is he thickness of the oil at any instant. Here V is constant. Only the radius and the thickness change with time.
So V/x = π*r^2.
Taking the derivative with respect to the time, we get
V(-1/x^2)*dx/dt = 2*π*r*dr/dt
dx/dt = -2*π*r*dr/dt*x^2/V = -2*π*r*dr/dt*(V^2/π^2*r^4)*(1/V)
Now simplify and find dx/dt