The engine will survive on just about anything in the 20 to 40 weight range. Heck, probably even 50 weights too.
What will work best (for the $)? HDEO 10W30. Rotella T5 is probably easiest to find but Chevron Delo, Pennzoil Long Life and others can also be found in 10W-30 if you look in the right places.
If you tend to torture your equipment, especially in 85F+ weather, I'd go with 15W-40.