Should I use 10W30 conventional (Castrol GTX) or 5W30 synthetic blend (Pennzoil) in my '99 Dodge Caravan (2.4L 4-cyl, 100k miles, drive it about 3k miles/yr, near New York City, rarely gets colder than 20F in winter). Bought the car new and always used 5W30 conventional (Mobil, Pennzoil) but now...