In a 4.6 liter 5w-30 oil with a viscosity of 66 mm²/s, adding 5% (approximately 200ml) of Liqui Moly Viscoplus, which has a viscosity of 299.6, I increased the viscosity from 66 to 70.50 mm²/s, which is a number between 5w-30 and 5w-40. The 5w-40 has a viscosity of 75.00, always at 40 °C.
The reason you can do this is because of the heat, which raises the temperature of the oil, causing it to thin out, with the kilometers you drive, it thins out again.