Mobil-1 15W-50 is one of the best you can use for this application - good choice. Fill spec is 7.9 quarts, but out of 8 quart bottles you'll always have a smidge left, so you'll be awfully close just dumping in 8 bottles. I never worry about that last tenth.
With M-1 you don't need to change before at least 5kmi, btw...
Good luck,