I hate seeing bad information floating around about motor and gear oils. Carl has good information, but has not touched on "Oil Shear". This was one, if not the problem, with late E36 M3's and into early E46 S54 engines. The problem was bearing failure and there was a recall by BMW to replace bearings.
That is why the 10W-60 oil is used. BMW figures that by the time you are ready for an oil change, the oil has already sheared down to maybe 40 weight vs 60. Conventional group III oils shear very easy, especially in a high reving tight clearance engines like the S54.
I have done a lot of independent research on lubrication and came up with AMSOIL being the best group IV oil on the market (since 1972).
Do your own snooping around and see what you come up with, but I highly recommend that you read what Bob the Oil Guy has to say.
Another good place to search is The Engine Oil Bible. All is excellent reading and NOT sponsored by Castrol, Mobil1, AMSOIL etc.
The question is do you want what is BEST for your investment? If you have a cheap Chevy or something, use the bargain oil. I have been running AMSOIL products for years now, and "I" know from personal experience that AMSOIL is a superior group IV product. Group V oils are not considered oils, but rather additives to the oil, or aftermarket additives or conditioners.
I am using AMSOIL 10w-40 (AMO) in my E12 Bimmer which is ideal for high mileage flat tappet cams. Has the Zinc needed. I used to burn (not drip) about 1qt of oil every 1K miles. I installed an AMSOIL (BK21) oil bypass system. What this does is take samples of your oil and filters it down to about 1-5 microns and returns oil to the sump, thus "cleaning" the oil from contaminants such as carbon, fuel, water etc.. We all should know that dirty or contaminated oil can lead to premature wear. And here is the kicker.. Dirty oil is consumed faster than clean oil. I have had my by-pass installed for over 1K miles, and the oil level is still full. Now I don't add that 1qt every 1K miles! (boy, was I surprised, and happy!)
On my '04 M3, I am NOT using Castrol 10w-60 TWS oil, but AMSOIL's 15w-50 (RD50) racing oil.
Look at test results for "shear" and viscosity among popular oils and you will see what I mean. Some say that the VANOS does not like 50w oil, but that is all they have to say to back up anything. Everyone says the Castro TWS 10w-60 is the best, but the only group IV oil that can be used on the M cars. TRUE IF you are under factory warranty, I would stick with it. I am no longer under warranty, so I choose to use a better oil.
If Budweiser was the official beer of NASCAR, does that make BUD a better beer? BMW uses the "European" Castrol 10w-60 because it is a group IV oil, made in Germany, which is different from the American counterpart, (Group III). BMW had to find an oil that would not shear and cause bearing failures, thus no more 0w-30, 5w-30 etc. oils could be used especially under warranty.
Once you understand how oils work, then you will see what Carl and myself are talking about!
I was so impressed with all the AMSOIL products, I became a dealer. I call them "designer" oils because they formulate their oils for specific applications. Using PAO synthetic stock, the chemical bond of the oil is stronger and less prone to shear vs any conventional or group III oil.
AMSOIL is also the only oil manufacture that can claim as a 100% synthetic, because it is. All the others are FULL synthetics and not 100% because they have a mineral oil base stock to make their oils. (ie. crude oil)
So, if you are looking for a quality product (made in USA not imported crude oil base stock) do some more research for yourself. I have already done it and expressed my views here.