5W-30 or 5W-40: reading the grade debate through a Gulf windscreen
Your owner's manual wins the argument. Most modern Japanese and Korean engines are designed around 0W-20 or 5W-30, and their pumps and clearances assume it; pouring in thicker oil because the weather is hot can reduce flow to precisely the parts that need cooling. Many German engines genuinely specify 5W-40 or a long-life 0W-40 meeting approvals like VW 502.00 or MB 229.5. Check for a GCC or hot-climate footnote, since many manufacturers publish a regional supplement that already accounts for the heat.
Where 5W-40 earns its keep is in older, high-mileage engines with worn clearances, where the thicker film helps oil pressure and curbs consumption. A competent technician asks about mileage and any oil-burning history before recommending anything; one who reaches for the same drum regardless of what you drive is telling you how the rest of the job will go.