Who chose a straight 40 weight oil?
The oil that was originally recommended in the Crossfire, along with just about every high end German car is 0W-40 oil, typically made by Mobil. That is a MULTIGRADE oil weight.
When the oil is very cold, it flows like a 0 weight oil.
When the oil is very hot, it flows like a 40 weight oil.
If you are using straight 40 weight oil in your car, you are the one messing it up.
Also, right in the owners manual, it clearly states that any synthetic engine oil, as long as it meets the MB 229.3 or 229.5 standards, can be used in the engine.
This standard includes 0W-40 and 5W-40 oils.
When you look at the engine oil bottle, it will typically say right on there if it meets one of the standards. If not, pull up the product data sheets on the oil company's website, and see if its listed there.
Here's Pennzoil's website:
Pennzoil - Not just oil, Pennzoil
You will notice under Key Benefits, it lists that their European Formula oil comes in a 5W 40 formula, and meets the MB 229.5 and MB 229.51 standards.
Here's Quaker State's Q Horsepower Ultimate Durability oil:
Quaker State Home
Their 5W-40 oil meets the MB 229.5 standard.
Castrol Syntec 5W-40:
http://www.castrol.com/liveassets/bp...syntec_usa.pdf
It meets the older MB 229.3 and 229.1 standards.
KendallŪ GT-1 Full Synthetic Motor Oil, SAE 5W-40:
http://www.conocophillipslubricants....0TDS%20Web.pdf
This one meets MB 229.5 standard.
Hopefully you are starting to get my point.
Personally, I would look for any oil meeting the 229.5 standard, as it's the most recent one, and use whatever oil I like the best from its advertising, as they are all going to perform just about the same under the normal 7k mile oil change interval.
BC.