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.

Good oil if you can get it but not sure it would be so great for a motor of that vintage.

IMHO a diesel oil with a lot of zddp would probably be better.

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2006 CL500
1969 280SE
2023 Ram 1500
2026 Genesis GV70
2007 Tiara 3200