The Universal grade oils are the ones that are formulated specifically for diesel engines. These are oils such as; Chevron Delo 400, Shell Rotella T, Mobil Delvac, Mobil Delvac One, etc. You will find these oils at the truck stop or over the road truck dealers. For that matter, I buy Chevron Delo 400 multigrade at Wal-Mart now. These are typically used in all climates in 15W40 grade.
Just because an oil has a C classification, doesn't mean it is universal grade. The universal grades have added detergents and additives for soot dispersal. These oils work quite well in gas engines as well. They will keep a gas engine quite clean inside.
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in