The beauty of synthetic is that it doesn't get thick or thin due to temps in winter or summer. I use Delvac 1 5W 40 on my TDI Jetta and go 10K miles. I read here though that these old Benz's put a lot more soot in the oil than the newer diesels so you probably shouldn't exceed 5K even though it is synthetic. So that makes me wonder if it is worth the investment for synthetic on these Benz diesels?? I suppose if you lived in a cold climate in the winter you could justify it for start ups and warm ups...other than that though??
I'm parking mine for the winter, so I'm porbably going to switch to delvac 1300 dino from now on and change at 3K (for the benz, not the Jetta)

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'85 300D - 206K (sold)
'81 240D - 149K (sold)
'03 Jetta TDI - (sold)
'79 300D - (sold)