I have a brand new vehicle that recommends 5W30. I am reluctant to use synthetics because of their $4+ price. Our local Walmart sells Mobil 1 ss 10W30 at $3.58/quart(other grades are higher). At 10 degrees F, which oil would perform better: a conventional 5W30 or Mobil 1 10W30? As long as I am not increasing start-up wear, I'm going to go against the vehicle recommendations and use Mobil 1 10W30. Opinions anyone? What is the shelf life of synthetic oil? If I purchased a quart of oil a year ago, how would it compare to the quart I would buy today if the oil was exactly similar (additives, etc.)? Thanks.