No need to use 0W30 but it's not wrong to do so. I use OW30 in 2 of our cars, but winters here go down to -30C regularly.

For my XJR I use:

AMSOIL SAE 10W-40 Synthetic Premium Protection Motor Oil

It has a -44C pour point, and is 40 weight for summer time heat, especially with the supercharger. I change it once a year. I used this same oil for a 1986 Porsche 911 for about 8 years without issue.

 

We had a lubrication Engineer from Esso come to a club meeting and gave us a presentation about oil. It's important to know what the numbers on the oil can mean: The first number is the viscosity when cold, the second is the viscosity when hot (100C), W means suitable for winter use.

His advice was to use the lowest first number possible to ensure the oil is pumped to the engine as fast as possible. Most engine wear occurs on startup, so it's vital to get the oil up into the engine as fast as possible. Thick oil when cold can't do this well. Even at 40C, a 20W50 is nearly 3 times more viscous than a 0W30 - 160cSt vs 57cSt.

20W50 is a holdover from the days when loose engines needed 50 wt oil when hot and the chemistry wasn't good enough to make a 0W50. There is absolutely no reason to run an oil with 20 as the first number today, in any climate.

He than said to run the second number as whatever number the car maker recommends. So if it originally called for 10W30, then 0W30 is fine. Heavier is also not better, nor is lighter.

Bearings in an engine are designed to have a controlled rate of leakage of oil, and the crank or rod journal floats on a film of oil and never contacts the bearing, except at startup.

If the oil is too thin, it leaks out too fast and the bearing is starved of oil and wear occurs. If the oil is too thick it won't flow into the bearing well, and what is there gets sheared excessively and creates a lot of heat. I've seen bearings melted from using too thick oil on industrial engines, and that also destroys the crankshaft journals. Aside from lubrication, the oil is also a coolant at takes away quite a bit of heat. I don't have exact numbers at hand, but on the industrial engines I worked on, it was around 25% of the heat rejected by the engine was taken away by the oil. It's a significant part of the cooling system, so having a heavy oil generating heat instead of cooling put extra stress of the rest of the cooling system. The engineers designed the leakage rate according to whatever viscosity they specify, I wouldn't want to stray from that. Oils have improved in terms of the chemistry, but the viscosity scale has not changed. Just because you have a fancy computer controlled thermometer doesn't mean that water boils at a different temperature, the measurement scale is the same.

So the motto is use either 5W or 0W (whatever the car maker recommends for the temperature you intend to operate the car in)

 

+1.

My wife's vehicle is a 2005 Chrysler Pacifica with the 3.5L V-6, bought new. It calls for 10W30, which even then was quite unusual but it's a rather high revving design. I did mineral oil changes every 5K until about 18,000 km then changed to Amsoil's fully synthetic OW30. The car has 240,000 km on it, I change oil once a year, and it uses maybe 1 litre in that time.

I tried Castrol 20W50 in my 86 911. There was a noticeable lag coming up to full oil pressure on start up. I couldn't believe how thick the oil was when pouring it in. I went back to 10W40 and it built oil pressure almost instantly.
The thickest oil I use is a 15W40 in a Dodge Cummins diesel.