How long can I actually go between oil changes on a 'extended performance' synthetic?

Manufacturer claims of 15,000–20,000 miles are real on paper, but the dominant view in r/MechanicAdvice and on bobistheoilguy.com is to stay closer to your vehicle manufacturer's OCI (often 7,500–10,000 miles) unless you're doing used-oil analysis. Filter capacity, fuel dilution and short-trip driving all shorten safe intervals well below the bottle's marketing number.