Non-expert armchair mechanic I have watched youtube videos of Mechanics claiming premature failure of Toyota engines (@200k) is due to 10,000 mile oil changes. They claim varnish from old oil builds up in the ring grooves causing rings to lock and wear cylinder grooves. Could they be wrong and cheap gas with Tars are causing this. I've changed plugs in cars that are driven in certain gas markets that are coated with Tar. Where the tar comes from, gas or older oil should not be an issue with the synthetic oil unless the engine runs hotter and the oil is turning Tan colored which I have noticed in my sisters Honda CR-V with Turbo charger but not in my Toyota RAV4 without turbocharger. Turbochargers are lubricated and cooled with engine oil so the oil gets hotter. Blow-by to a certain extent is normal and low demand mileage driving allows buildup of gas in the oil which is why specs call for 6 month and or 10,000 mile changes. In older engines with worn rings, carbon blow-by also causes the oil to turn black. So rule of thumb taught me by my father is to change the oil when it turns color (black or TAN at about 3,000 miles) AND/or 6,000 miles on newer cars with still clean oil. Black means excessive blow-by which can have excessive gas in the oil, Tan means the rings are good but the oil has been heated past it's safe operating temp and is breaking down.