The 6,9 is not built for synthetic motor oil and it can do real damage. I hunted a rumbling sound in a 6,3 engine once and the only cure was changing the oil. I used this: http://www.kroon-oil.com/en/products/catalogue/seg_aut/krmo/62/automotive/kroon-oil-motoroils/bi-turbo-15w40/

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600SEL '91
300E 4Matic '88
240D '83
280SE '77
350SE '73
The most complex systems can fail in the simplest way.
Contra verbosus noli contendere verbis, sermo datur cunctis, animi sapientia paucis.

i don't believe in the lord! He's never bought me a Mercedes Benz.