Booking on price alone
A quote well below the market range for a genuine synthetic is not a bargain, it is a disclosure. The savings usually come from decanted or counterfeit oil, an unlicensed operator with no insurance, or waste oil that will never see a licensed collector. Compare like-for-like itemised quotes and let the outlier go.
Paying without paperwork
A cash payment with no invoice buys an oil change and nothing else: no warranty evidence, no resale history, no recourse. The invoice with company name, TRN, odometer, oil specification and filter part number costs the provider two minutes and is most of what you are paying a legitimate business for.
Upgrading to thicker oil because it is hot
The instinct that 45-degree summers need heavier oil is decades out of date for modern engines, which rely on precise flow through tight clearances and variable valve timing. Going thicker than the manual permits can starve exactly the components the heat threatens; the GCC specification already accounts for the climate.