with time our education/knowledge grows....
IF your temperature needle is in control and coolant is inside for better cooling...
needle works on thermo switch, so in winter & summer it would be in middle for same temperature say 50 degree.
you can use good synthetic like 5W-30 (Caltex) or 0W-30 (ZIC).
Car in subject is CITY, so the owner should look for Mobil-1 5w-50. or 5w-40 (ENOC, ZIC, Shell)
original Mobil-1 10w-40 is full synthetic as it costs 2600~2800.