Two choices: Synthetic (any good brand) or Pennzoil Multigrade dino if you want a less expensive option. Either should work fine. Pennzoil's 5w-30 Pour Point is an impressive -44F, while the next best popular brands are Kendall GT-1 at -40F, and Valvoline All Climate, Mobil Drive Clean, and Union 76, all at -38F. On the other hand, Quaker State Peak Performance is -27F. Pennzoil 5w-20 also has an edge over its competitors, while Pennzoil 10w-30 does not.
While many brands don't list their Borderline Pumping Temperature, Pennzoil 5w-30 does, and it is -31F. So you'd be protected well below zero F with this oil.
Keep in mind that in consistently cold weather (even above 0 degrees F), the pour point depressant additives are used up fairly quickly. As these additives are used up, the pour point of the oil rises. In addition, after the engine heats up, the oil is subjected to high temperature conditions that burn off the lighter components in the oil, causing it to thicken, further increasing the pour point. Therefore I'd shorten your winter oil change intervals well below 7,500 mi or whatever GM is recommending.
FYI...
Pour Point: This is 5 degrees F above the point at which a chilled oil shows no movement at the surface for 5 seconds when inclined.
Borderline Pumping Temperature: A measure of the lowest temperature at which an engine oil can be continuously and adequately supplied to the oil pump inlet of a passenger car engine, with the oil pump maintaining adequate oil pressure.
Cold Cranking Viscosity: Simulates the viscosity of an oil in crankshaft bearings when trying to start during a cold winter morning. The test is important in determining if an engine can be cranked over fast enough, when very cold, to start.
Conventional oil spec's:
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[ December 03, 2003, 02:29 AM: Message edited by: TC ]