Ever since I got my Saab I've been scratching my head about this. My Saab 9-5 is the first car I've owned that's called for 0W-40, every other car I've had called for a 5w-30 or a 10w-30. My question is this, it just seems like 0W is the best because it'll flow quicker to all the parts when the cars started. So why don't all manufacturers use it? Why is 5w or even 10w still a thing? My Saab calls for 0w-40 for quick lubrication upon start up and 40 weight when hot to keep that turbo lubed up. Just makes sense.