Castrol 10w 40 is light for the summer in a diesel, we used 20w50. If you live in the cold climates 10w40 is good for the winter. Both meet the mercedes Diesel requirements for the 80's models. I would however recommend 20w50 for summer. ( quarts seems high. I believe jgl1 is correct about it being an 8 quart fill.
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)