The most important life factor is operating time at low temperature for both oil and filter. In the winter, you need about 15 minutes driving at moderate load to get the oil up to operating temperature if the thermostat is working properly. It is preferable to operate for a minimum of 30 continuous minutes at least once a month if you are only driving it occasionally to allow the oil enough time at a high enough temperature to evaporate the condensed water, acids and fuel. If you can do this and use an upgraded filter that contains synthetic enhanced cellulose filter media (like Wix, Napa Gold or Fram Toughguard for example), then yearly changes are acceptable but watch that dipstick for any signs of moisture. If you cannot do this, better stick with the 6 month intervals. I don't think that synthetic oil helps extend the oil drain interval if it must be dumped because of contamination. As a fellow Idahoan, one more bit of advice, "Never put your horse away hot and never put your car away cold".