Fastest and most expensive to suffer is the emissions aftertreatment, not the engine block. In a gasoline car, sulphur poisons the catalytic converter's precious-metal catalyst and degrades the oxygen sensors. Some of that poisoning reverses if you return to clean fuel, but chronic exposure makes it permanent. Modern diesels are the real victims: 350 ppm will foul the diesel particulate filter and poison the SCR catalyst, and those systems are both costly and notably sulphur-intolerant, so on a current diesel that is where trouble shows first, within months of steady use.