I have a car that I only drive 200 miles a year. I put fresh Mobil 1 10W-30 in it 18 months ago and have driven about 300 miles since then. How long can I safely leave this in at this rate. Should I change the oil at some point just due to age? I know I could get an analysis done, but I'd like to know more about the theory of deterioration of oil and additive packages and how that affects shelf life. Thanks!