Been too long since I looked up the exact figure (measures in litres) but the 81 SD uses just slightly less than 8 qts of oil.
I use the Mobil One Truck & SUV 40-10, but do not run it for any longer than 7500 miles. Too much soot on this model engine to run safely any longer than that. Best place to get it now is at Wally World. They sell a 5 qt bottle for $19.97.