Bob: I note you're in Charlotte. My excellent independent -- Earle Whitton at Charlotte Star Service, 4225 Monroe Rd (just south of Wendover); tel 704-332-4111 -- just did a change of the differential oil in my daughter's 190E [I also have an '88 420SEL] last week and at my request put in synthetic oil.
If you choose to use Earle, he can get the synthetic for you (be sure to tell him that's what you want); for the 190E 3 units of the oil was $27.45 and the labor was $32.50 -- prices for a 420SEL might be slightly different.
Hope this is helpful.
'98 C43 AMG
'93 190E Limited Edition (including factory Sportline package)