If a person starts to extend oil changes beyond 3000 miles (using a good oil like Schaeffer's), are there any problems with acid build-up or other harmful chemicals that cannot be removed by the oil filter? A mechanic once told me that even with synthetic oils, beyond 3000 miles there would be enough acid build-up and other chemical contamination build-up to make necessary an oil change. Perhaps this would only be true if a person was doing mostly short trip driving. Also, I personally have never gone much beyond 3000 miles for oil changes. But I noticed in another post that Gary Allan said that whenever he went past 4000 miles on an oil change, regardless if he was using Mobil 1 or a conventional oil, his oil consumption would increase greatly. I found it interesting that he said that this happened even if he was using Mobil 1. Do motor oils start to break down after 4000 miles?