Sorry to be of little knowledge but what is 0W-40?

Is it lighter, heavier, for high mileage? I am assuming it is synthetic

I did find 0W-30 is that similar?

Thanks

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1993 400SEL (W140) 281K
1999 S500 (W140) 163K (Sold)
2002 S500 (W220) 72K
1999 S500 (W140) 88K
You cannot simultaneously prevent and prepare for war.
Albert Einstein (1879 - 1955)