Hit Man X
Just completed oil change on my '87 300TD the other day and used Mobil 1 5W 30 for the winter. Drained oil was Mobil 1 15W 50 for the warmer months. No particular reason for my selection other than I happen to use Mobil 1 throughout my vehicles. The '84 300D Turbo will be seeing the remainder of the Mobil 1 products soon. Plan to use the same viscosity scheme. Choose your weapon and believe in it.
PS (Search feature admonition gets all of us Newbies . . . I got my justified "slap, slap" a few times. Grin and take it because these folks are seriously great)
Best