The official reply from Royal Purple:
Good Afternoon Mr. Rascal,
For the ZF 6-speed in your F350, we do recommend Synchromax. If yours has a factory oil cooler on the transmission, it takes 6.35 quarts. Without it takes 5.8 quarts. If the packaging is still in good shape and the bottles have remained sealed, the oil should be perfectly serviceable. Just give each bottle a good shake as there may have been some settling over years of sitting still.
Thanks for choosing Royal Purple and have a great day!