They are underneith the valve covers.
The 7.2 liter is a Duramax diesel.
On A Sprinter it is 12.5 Litres
Your injectors are failing Also, could be the turbo seeping oil past the bearings.
12 quarts with filter
Approx 12 quarts
13 quarts of 5w40 synthetic diesel engine oil.
With a 48re approx 15 quarts With a 68rfe approx 17 quarts with a AS68RC approx 13 quarts