I really like that Rotella 15-40;I used it in my Mack when I changed the oil,and I'll be putting it in my GMCs too. I don't remember the particulars about it,but it's one of a very few oils that has additives that protect the cam lobes etc. from excessive wear. It's like it was designed for our old trucks. BTW-when I was in Reno last time,I found it at AutoZone in gallon jugs,and the price wasn't bad. Here in Elko,I'd check NAPA or Checker for it.
Speed

1954 GMC 350
1957 GMC 1/2 ton
1962 Chevy C-30
1952 Chevy 6400 dump bed project truck
'98 Harley FLSTC
'66 Pontiac Catalina
'76 Chevy 1 ton Duallie
'84 Bronco II
'78 Dodge W-200
'81 Toyota 4X4 truck