The reason for oil consumption is not thinner grade. It's due to other major reason which may require overhaul. For example, stuck oil rings in piston due to excessive carbon or damaged valve seals. It could be as simple as a bad PCV valve. Oil reduction issue never happens if you keep changing the synthetic oil at 8000 km or 6 months, which ever comes first.
In reply to oil consumption in your old city, I used 0W-20 in my 2015 Wagon R during my ownership, which lasted 6-7 years and 80,000+ km. I did multiple trips from Islamabad to Karachi via motorway. I usually stop at Sukkur, but I remember doing one trip non-stop for 18 hours with speeds 110-130 kph during summer with temperatures exceeding 40C during the day. At the end of the trip, not even a single drop of oil was reduced. No overheating during the whole trip and engine performance remained the same till the end. This was done when the vehicle's mileage was above 50,000 km.
Highway driving is not intensive at all on engines. In fact, it's less intensive than city stop and go driving. It can get intensive if you constantly over speed for long duration. The limit for over speeding varies from vehicle to vehicle.
As a proof, I suggest you to perform some live monitoring on engine temperature when driving on highway. In my experience, it never exceeds 90C. But in city driving, it does exceed 90C and fan turn on frequently.
True for racing. Racing oils are true synthetics based on PAO and are thicker than normal oils that we use. Excessive wear occurs if thin oils are used for racing. I never found any reference for highway driving.
