Temperatures in Greece often go over 40C in summer and when stuck in traffic under the sun things could get even worse. Its not a big difference and i prefer to be on the safe side. 10w40 is recommended for a bit cooler climate than Greece as the UK where some might even go for 30.
In hot weather, you will need oil motor oil of a higher viscosity, because the heat will interplay with the engine to have an impact on the oil. People living in hot areas or changing their oil before the summer should think about oils in the 10W-50 range: the “W” means that the oil is suitable for winter use as well. In cold weather, low viscosity oil is needed, because the cold will make the oil sluggish—10W-30 or 40 is more appropriate. For people in temperate climates, 10W-40 oil is ideal.