The "W" specs for engine oil are tests conducted at -25C, -30C, and -35C (-13F, -22F, -31F) for 10W-xx, 5W-xx, and 0W-xx oils. It doesn't matter if it's synthetic or not, if the oil is labelled as a certain "W" number, it will perform at that temperature. How cold does it get in KY?
The relevent tests are Cold Cranking Simulator conducted at the temperatures listed above, which measures the effect of the cold, thick oil on how fast the engine can crank to get started (not really an issue with fuel injected engines, but big deal for carbureted) and the Mini-Rotary Viscometer, measured at a temperature 5C lower than the CCS test, which measures yield stress. Basically you can think of yield stress as sucking a hole in a milkshake - the fluid is too thick to slump or run and fill the hole fast enough and the oil pump ***** in air.
I've used 15W-40 diesel engine oil through winters in NY (never gets below about +5F) and while it'll crank slow, it'll run fine. 74,000 miles and counting...
May I suggest a 0W-40 (usually listed as a synthetic European passenger car oil formulation) or 5W-40 (diesel synthetic) as very good choices? Or the usual 5W-30 vintage?
Many times over it has been documented that pour point does not correlate to engine starting performance, even though it is often shown that some oils have pour points below -50C. It doesn't matter to the engine.
