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