5W-30 or 5W-40: reading the grade debate through a Gulf windscreen
The commonest technical argument at a UAE oil change is grade. The number before the W describes cold-flow behaviour, nearly irrelevant where a cold start means 28 degrees. The second number is the one that matters, because it describes viscosity at operating temperature, and Gulf engines live at full temperature in 45-degree air, idling in traffic with the air-conditioning compressor loading them.
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.