It has a 4 valve per cylinder head and a 10.4:1 compression ratio. I think that would equate to a high enough cylinder pressure to warrant premium. Otherwise, the engine may retard the timing to prevent detonation with lower octane gas.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record