Also, I've been given some totally bullsh*t answers from people here in the past so I'm going provide some reasons why they're wrong.
TLDR Version: Oil viscosity can vary greatly. Some 5W30 oils are far thinner/thicker than other 5W30 oils etc. They are not all the same, there is a certain tolerance before an oil becomes considered a 20 weight or 40 weight etc. Some 30 weight oils are close to the upper limit of 20 weight, some 30 weight oils are close to the lower limit of a 40 weight. Some are right in the middle!
I've been studying the bobistheoilguy forums for around 12 months now and learned many facts. I'm still a n00b compared to some of the chemists and highly qualified people who work in the oil industry. I'll try keep this brief and recommend you all read and join bobistheoilguy.com to understand more.
First thing's first about viscosity - the number on the front of the bottle 5W30, 10W40 and so on, is just a category that the oil met the specs of. So some dumb things I have been told are "Use XXW30 weight oil. Our cars spec 30 weight, that's all you need, 0W30, 5W30 and 10W30 are all the same at operating temp" INCORRECT! Reason this is incorrect is that the oil viscosity can be a wide margin apart from brand to brand.
Example 1:
Penrite 5W30 Full Syn here in Australia (API SN)
Viscosity, Kinematic, cSt
at 40°C - 52
at 100°C - 9.6
Example 2:
Castrol Edge 5W30 Full Syn (available in most places) (API SL A3/B4)
Viscosity, Kinematic, cSt
at 40°C - 70.66
at 100°C - 12.02
What does this mean? Well the first number is a test done at 40 degrees C and the second at 100. So it's somewhat of an indicator the oil's viscosity when cold (40°C) and when hot (100°C). The penrite is so close to being a "thick" 20 weight spec oil and the Castrol Edge is closer to a very thin 40 weight spec. So the old blanket statement of "use only 30 weight oil is not entirely correct. Saying that some 40 weight oils are too thick is wrong too, as one particular brand may be close to the lower limit that 40 weight allows.
The next silly thing I see is this: "0W40 flows as well as 0W30 when cold. They are both 0W so they are the same when cold." VERY wrong. As I posted above, the viscosity on the label is just a specification that was met. 0W20 will ALWAYS be thinner than 0W30/40/50 etc. In fact, the oil can never be too thin at start up. It has not happened yet. There are some oils with KV cSt in the 40's at cold but remember, when hot at 100°C it is always thinner than the cold KV cSt at 40°C.
Hope you're all still with me here
Without going too much deeper on this subject, there is also a HTHS spec. This is a high temp high stress test at 150°C. At such temps, the numbers drop even more. This is a good indicator of how the oil stands up to abuse. Think turbo or race conditions. Think about fuel dilution or oil shearing (thinning down). Oils with higher HTHS won't thin out as much in such conditions. The last thing you want is to buy an oil with a weak HTHS that is on the thin side of 30 weight, that then shears down to a mid range 20 weight oil! 20 weight is too thin for our cars. A user on the BITOG (bobistheoilguy) site posted used oil analysis on his KLG4 2.5L V6 and the 20 weight oil had more wear metals in it than the 30 weight tests he did. I've also seen a number of Subaru WRX UOA's and their manual states the use of 30 weight oil. Many of them have had their 30 weight oil thin right down to 20 weight after a few thousand miles. Plenty of forum users like to use Shell Rotella 5W40 oil on their WRX as it does not shear as much and their UOA's prove that going to a thicker weight produced less wear metals and worked for more miles before depleting the anti wear additives in the oil.
Moral of the story here is to research more and don't say 40 weight oils are too thick. Because you don't truly know how thin or thick the oil actually is without knowing the 40+100 KV cSt and HTHS numbers. Avoid "fuel saving" oils as they are usually on the thin side of 30 weight and could shear down to a thin 20 weight in a bad scenario.
One last piece of useful info I have found. API spec and ACEA specs. You've seen API SN/CF on most newer gasoline engine oils. There's also ACEA specs such as A5/B5 A3/B4. Simply put, I ignore API SN and look at oils with ACEA A3/B4. The A3/B4 spec have a specific HTHS strength and they usually have a TBN (additive level) of 10 or more. TBN of 10 or more lets you run more miles on that oil before requiring an oil change. Some oils have TBNs of 6 or 7, which might last much past 5000 miles. It's just peace of mind that the oil won't lose it's additive package and turn to sludge unless you put some seriously high mileage onto it.