moss,

the 15w40 will be fine down to 5 degrees Fahrenheit. if it gets colder than that, 10w40 will be good down to -4.

mercedes recommends 5w50 for all temps for your E320...which would mean switching to synthetic.

you'll probably get about 8 million different opinions but i'd say you're fine sticking with the 15w40, that is unless you see temps below 5 degrees.

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