The blue bottle T6? It is in my SD currently and my PSD, I do not drive distance enough to warrant extended drain intervals in the SD so I just change it at six month intervals. March and September. Biggest plus for the T6 gave me quicker oil pressure on cold starts this winter. Not sure if that is due to the 5W versus 15W, but probably. I am kind of sad the significant upgrade in cost for the T6 is just a highly refined dino based oil. :/ It will make me think twice about its purchase next time unless on sale.
Usually I just use whatever oil is on sale.
SDL has Delvac I think, maybe Delo. 300SEL Walmart 15/40 Diesel, seeing how it would treat me after my valve job. I assume it will be comparable to any other dino based 'Diesel' oil. 560 has M1 15/50.
I could see a slight advantage of syn in a volatile climate such as AZ/NM/TX in the summer, but probably not enough to warrant spending twice the money for the same oil change intervals on these cars.