Don't remember the specific numbers (moisture, amount of remaining detergents/additives, metal presence, etc.) but the gist was that for my vehicle at the time, 15K intervals shouldn't be a problem. Though I often hit 1 year before I hit 15K miles, so I change then. I haven't gotten an analysis done at one year to see if I could go more than that time-wise (things like moisture, acids, etc., are what I'd be concerned with past 1 year).
Another vote in for 1 oil change/year being cheap insurance if you don't hit the mileage limit.
It was also written with whatever type of oil the car shipped with in mind. If the car shipped with conventional and the owner has switched to synthetic, the interval is going to be longer. My van shipped with synthetic, and the manual specifies synthetic and 10K intervals, which I stick to.
From here:https://www.mobiloil.com/USA-English/MotorOil/Car_Care/AskMobil/Recommended_Oil_Change_Intervals.aspx
Recommended Oil Change Intervals for Mobilâ„¢ Motor Oils
You state that your oils are for 5,000, 15,000 miles, but what are the months before a change is due?
-- Billie Baskett, Robinson, TX
Answer:
We recommend you follow the oil and filter change frequencies shown in your owner's manual. Generally, this means changing your oil every six months when using Mobil Superâ„¢, Mobil Super High Mileage and Mobil Super Synthetic motor oils, and every year when using Mobil 1â„¢ or Mobil 1 Extended Performance synthetic oils.