I would be a little cautious about using a diesel oil in a gas engine. Car and Driver's Pat Bedard had an editorial on oils a few months back. It seems diesel oils have additives that could damage catalytic converters. The E320 is a LARGE car with a small engine. Depending on how you drive I wouldn't stray from 20-50 convetional or 15-50 synthetic in the summer. I left the 20-50 in my wifes 740 BMW until after Christmas this year. We live in central Minnesota and it gets cold! When it came due for a service I switched to 10-40. Seems to turn over a little easier when its below zero. With the miles you drive I would go with 10-40 over the winter and 20-50 for summers. Change it twice a year, spring and fall. This will avoid any contaminant build up from lack of driving too.
90 300GE 5sp