...to the MB Diesel world!
As mentioned above, regular oil changes are a must. Do a search on the subject of oil, and you will find a plethora of information as to viscosities, and types. One thing that may have been overlooked, is the fact that neither the D nor the SD have hydraulic lifters...the valves must be adjusted every 15K miles according to MaBenz. (this may be of some aid in cold start, and performance issues in your cases)
An additive I'd like to recommend is RedLine Diesel Fuel catalyst...its a good thing all the way around (read the bottle and you'll know what I mean...there is also a link to their site from here, explaining its use)
Dr. G, how many miles on yours?
I think you will find that almost everyone here agrees that the 617.95x (5cyl diesels) are quite the workhorses and will give hundreds of thousands of miles of service.
(the 4 cylinders are also, but neither one of you has one.
)
Those were my welcome, and my 2¢
-Larry