Back when I did my first synthetic oil change in the car, I used 10w30 oil ( recommends 5w30 in our car; I had it on hand at the time). I've been driving the car all winter long using this oil without problems, but the engine seemed to run rough and shift hard until it was completely warmed up.

I did another oil change a couple days ago, but this time used 5w30 as recommended. The difference in cold weather is day & night. The car drives much better immediately after a cold start, makes less noise, and seems to get up to temperature quicker.

I really didn't expect 5 cold-viscosity to make such a drastic difference.

 

I'm in the "these days, there's no reason to run anything heavier than 5w" camp...in my A4 and GTI I ran 0W40 (which was recommended in the manual) though the majority of the difference between 0w and 5w is all placebo effect in my head lol

Depending how cold it got in Wi, I can believe you'd feel the difference though till it got up to temp