I was just wondering what exactly is the reason for changing out the oil in a year time frame? I'm just curious. My mother-in-law only puts on her car about 4k a year. She is running the 5w-30 Amsoil. I know that the oil is still way good. I understand that her driving, which is mostly around town top and go and occational highway runs isn't very stressing on the oil and the fact of condensation is the biggest factor. Is the one year just a good calender interval to go by? Any reason the interval couldn't be extended since the mileage is so low? I'm just curious. Thanks oh if it makes a difference she has the 4.6l DOHC V8 in her 95 Mark VIII Total miles on the car is right around 29k.