Oil simply cannot maintain 100% film strength, anti-wear, etc. From thenfirstmcrank after a change, the oil begins to degrade. The better the oil, the longer the protective properties hold out. But by 5%, the oil is on it’s last legs and most assuredly is not offering the protection and lubrication it once did. That being said, the oil change indicator on most vehicles are a rough estimate of oil condition based on milage and load (best case), and many simplyncount miles…but it accounts for a little (very) margin.
Don’t run on oil until 0%.