How many quarts of oil does a Volvo VNL 780 use?

That depends on if yours has the Cummins ISX, VED12, VED13, or VED16. I believe there was a VNL780 in production when the Detroit Diesel 60 Series and Caterpillar 3406E, C12, C13, and C15 were available options for Volvo, as well.