Personally, I like the additive package in the Valvoline line. It seems to have the best detergent qualities. It's all I've ever used and every Valvoline engine I've had apart in different degrees was clean enough to eat off of inside. Mobile One makes my engines leak but I believe it works excellent for sub zero start up and Rotilla-T, while and excellent product, doesn't seem to keep the engine near as clean inside as the Valvoline products. The Valvoline Premium Blue and Premium Blue Extreme 15W40 is their best diesel product.
http://www.valvoline.com/premiumblue/
You live in California by the looks of your post, Synthetics are nice in the cold weather for start up protection, but for the money, the advantages are questionable if you are changing oil every 3-5K anyway and operating in above freezing temps all the time. The most important aspect is to ensure the oil you choose is API Diesel rated CI-4 Plus, CI-4, CH-4, SL--(gasoline rating) is not important. The engine is 20 years old, ensuring a API diesel rating is the most important factor.
Your owners manual outlines oil change qualities, the 603 requires 8 Liters with a filter and 7.5L without. 8 Liters is equal to 8.45 quarts.
IMHO, there is such a contaminate load in the oil of a diesel to start with I would have a tough time leaving it in place for the miles the oil manufactures would like. Browse this link on Motorcycle oils. It's fascinating. Yes I know it's not an MB diesel article, but I think it reveals some very interesting data concerning viscosity retention. Kind of an eye opener.
http://www.xs11.com/stories/mcnoil94.htm