Different oil weights are not as critical with a synthetic oil. Due to the superior ability of synthetic to maintain stability no matter wha the temp. Many folks use 0- 40 all year round even summer heat with no issues . Unless you live in the desert where temps can be well over 100 degrees a 5- 40 full syn will suit from -20 to 100 degree summer heat and still have better protection than any dino oil. Syn oil is designed for temp extremes. With dino oil it becomes more critical to match temp to oil weight. To answer your cold temp question, my 450i has saw temps of -12 degrees and have had no issues and temps of 102 in the summer when the black seat gets to hot to sit on and still performs well. Like everything else, all functions get stiff in the cold temps! I will say the notchy shifter problems got better after switching to syn oil..so keep that in mind
