I'm running 15W40 Delo 400LE diesel oil in my stuff. Supposed to be a little better than Rotella; wouldn't guess it matters, though. I would bet money both superior to "high mileage" light duty oils, which would serve you no purpose, IMO. 61,000 is pretty much a new engine.
Some of my crap:
1963 C-30 flatbed dually 292/SM420
1965 C-10 LWB fleet 250/column shift 3 speed
1965 C-30 9' stepside 250/SM420/4.10 gears (my DD)
Website I made for my crap:
http://www.66submarine.com 
