I have an old Volvo with 200k miles(2.4l NA) and I use full synthetic oil (M1HM) and change it once per year (about 5000 miles-don't drive much) Hasn't burned anything in the last 5k and blowby is minimal. I live in Minnesota where the coldest I have had to drive this car in was about -10 F and the average summer day is 70 F to 85 F but there are times when it can get over 90 F. The first time I changed the oil I put in 10W-30 but upon looking at the viscosity chart in the owners manual it says that 10W-30 is only good until -4 F and 5W-30 is only good until 86 F. My question is can full synthetic 5W-30 and 10W-30 last beyond/below the temperatures in this chart or do I need to switch between seasons or use 5W-40. This doesn't seam right I thought full synthetics could withstand extreme temperatures?