As noted in the past, all automotive engine oils are judged/evaluated by the API, American Petroleum Institute. Though Mobile 1 and AMSOIL have very close to equal specifications, AMSOIL bests them in most tests used by API to rate automotive engine lubricants. Mobile 1 is a good quality lubricant and enjoys a large following from automotive enthusiasts.
AMSOIL has recommended a 25K/1 year drain interval since 1972. The only codicil being 15K/1 year for severe service, such as, heavy duty towing, dusty/dirty or long periods of idling being the only restrictions. Mobile 1 touts the 15K drain, yet makes the severe service a reason to return to vehicle manufacturers drain intervals or the vehicle Oil Life Monitoring system recommendations. That is no where near the 15K/1year touted in their ad.
My conclusions are that Mobile 1 is a good product, and could easily give good service for 15K/1 year, yet Mobile is reluctant to follow AMSOIL's lead and remove the restrictions and guarantee the lubricant for the full 15K/1 year period. AMSOIL has been around longer than Mobile 1 and is still recommending 25K/1 year change intervals on our Signature Series lubricants. AMSOIL is the leader in synthetic automotive lubricants, move up to the real #1.