Most people would agree synthetic oil is superior to dino juice. I think it is really a question of being cost effective and that depends on the individual. I run synthetic in my ML and change it at 7500 miles. On the other hand, I run Shell Rotella in the E300 and change it every 5000 miles. Why the difference? The way the cars are driven is why. I drive the E300 about 2000 miles a month and my wife drives the ML about 500-600 miles a month. That oil stays in the E300 about 3 months and in the ML for about a year. I chose to go synthetic due to the time interval between changes in the ML.

As far as testimonials with diesels, what do the truckers use? They might be a good source of info.

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99 E300 133K
98 ML320 99K
87 Saab 900 Turbo Conv 123K