You might be interested to know that Volkswagen's newest CNG cars do exactly that - there's a multi-year countdown configured at the factory, and when it runs d

You might be interested to know that Volkswagen's newest CNG cars do exactly that - there's a multi-year countdown configured at the factory, and when it runs down you can't use that fuel anymore, just the gasoline fallback! That's particularly "fun" here in Italy, where (unlike in the rest of Europe), metal cylinders currently need to be removed from the car and sent to one of two national inspection centers for high pressure testing - which requires them to be empty, of course! And unlike the oil change reminder, you can't reset it with just a button press, you need a computer interface (for which there's a fair amount of competition, but the cheapest for personal use seems to start at 60 € before shipping and is a "cloud" service)