When I refer to universal grades, that is the term for the oils developed and marketed specifically for diesel engines. Examples of universal grade oils are: Chevron Delo 400, Shell Rotella T, Mobil Delvac, Mobil Delvac One, etc. If you go to a truckstop, these are the oils you will find. Typically they will be in 15W40 grade.
If an oil has a C rating, it is not necessarily a universal grade, developed for diesel engines oil. BTW, these oils work quite well in gas engines. Their added cleaning agents will keep a gas engine spotlessly cleaned internally.
'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