The 5W part of the oil description means the oil does not exceed specified maximum viscosity limits at -30*c (-22*F) and -35*c ( -31*F).
ANY 5W oil (5w20/30/40/50 ... ) performs ROUGHLY the same at the measured temps. If it didn't, it would not receive the 5W rating, and would instead be a 10W, or 15W etc. Your expected winter temperatures are MILD ... you could run almost any multigrade oil without a problem.
A 10W rated oil meets these same maximum viscosity limits at -25*c ( -13*f) and -30*c (-22*f).
A 15W, at -20*c (-4*f) and -25*c ( -13*f)
A 20W, at -15*c ( 5*f) and -20*c ( -4*f)
For EXTREME cold temps ... consider using an 0Wxx oil.
An 0W rated oil meets these same maximum viscosity limits at a VERY COLD -35*c (-31*f) and -40*c ( - 40*f).
The cold temperature tests are below:
Cold cranking viscosity simulates the viscosity of an oil in crankshaft bearings during start up on a cold winter morning. The test determines if an engine can be cranked over fast enough to start under extreme cold ambient conditions. ASTM Method D 5293 simulates an oil's cranking resistance when cold, and thus indicates the lowest temperature at which an engine is likely to start.
Cold pumpability measures the resistance of an oil to pumping through the engine after a cold start. The most widely used test is ASTM D 4684. If an oil's viscosity becomes too high, pumping will be hindered with possible cavitation issues. Viscosity here becomes an important factor in determining whether the engine runs with sufficient lubrication after starting in severe cold conditions. The Cold Pumpability test is always conducted at 5°C colder than the Cold Cranking test to ensure the pump can deliver the oil to the bearings.
