If it is pre-2007, you can use Mobil 1 0w 40. This is what I use in my 95 E300 and 2005 E320 CDI. Meets MB 229.5 which is the standard the pre-2007 CDIs were built for. Original OCI called for 13k miles on CDI under "normal " drving conditions. I change between 10k and 13k as mine sees mostly highway and long hauls cross country. E300 gets 7500 mi. OCI which is what manual calls for but that was assumed to be using traditional oil. Probably could extend that with the synthetic oil.

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-- Chris

'95 E300, 216k miles, Silver Surfer
'05 E320 CDI, 138k miles
'07 S550 4matic, 69k miles

Gone but not forgotten:

'76 300D, 350k miles?, SOLD in 1995
'75 240D, 300k miles, SOLD in 1991