Which grade oil for a car that is not driven in winter in Chicago?

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1.
I understand from the dealer that they recommend 10W-40 for winter and 20W-50 for summer.

The manual that came with the car recommends 10W-40 for ambient temps of 0F to 90F and above.
The 5W-50 that the manual recommends for below 0F upto 90F and above is not widely available.
The 5W-30 per the manual is useful below 0F but protects only upto 50F.

I guess for a non freezing temps driven car, 10W-40 is the only one and is excellent.

2.
On my Ford Taurus, the Ford manual recommends 5W-30 oil all year round. I guess one car's (Ford's) meat is another car's (Benz's) poison (atleast in temps above 50F)!

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Well I talked to the dealer.
He says they use 20W-50 oil for Chicago summers (from April to Sep)
He says they use 10W-40 in winter.
Reasoning:
Some baloney about 20W-50 since "these engines run hot".
I know that oil grade has nothing to do with engine cooling. A heavier oil will quieten engine noise and seal some minor leaks.
Wonder why MB would state in the owners manual what I have said in my previous post!

[JNLeChevet]

You don't mention the year or engine type - gas or diesel?

Your Ford Taurus is probably a newer car which was designed for a lighter oil which improves CAFE for Ford. Mercedes is less concerned about boosting their MPG than building an engine that lasts (I hope!)

Stick with what your owners manual calls for. I personally do not like to use 5W-50 becuase of the added viscosity extenders and modifiers required to meet the higher viscosity rating. These will degrade as the next oil change approaches and the 50 will be more like 30 and there will be more deposits from the modifiers and extenders than with a 10-40 oil. If you really need the extended viscosity range, run synthetic. You can run a synthetic like Shell Rotella 5W-40 year round. 15W-40 (dino) would be a good summer oil. 20W-50 dino seems a bit on the heavy side, but should do no harm. Just make sure the grade (S or C as appropriate) meets the manual's specs. If your manual calls for SH, make sure SH is listed on the can even though it meets the improved SJ spec.

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4.2 liter V8 gas

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1993 400SEL