It's safe to assume "mechanics" and "friends with mashwaras" don't know what they're talking about 99% of the time.
10W is the cold rating, 30 is the hot rating. 10W-30 means it's a multigrade oil. If it said just 30, for example, that would be a monograde oil.
In Karachi, for example, 0W-30, 5W-30, 10W-30 all behave similarly. It doesn't get cold enough throughout the year for the cold rating to matter. All those oils meet the specifications for an SAE 30 oil at 100°C.
Your owners' manual is correct and you should always follow what it says.