Funny this came up. I thought about asking the same question a little while back. My '97 PSD has always had a diet of Delo but for some reason, I couldn't find any last month at WM. I found it at O'Reillys, but it was 18 bucks a gallon and they only had two. The guy wouldn't price match either since all I could show him on the Delo was the "internet" price on my phone. I explained that the price is what they sell it for in their store, but no dice. So, for $10.74 a gallon, I got the ST 15w-40 and figured 3k miles couldn't hurt anything. I changed oil/filter and drove the truck for 200 miles before a bearing spun and sheared the crank!!
Just kidding, but seriously, the truck has been running just fine, is getting 17 mpg (up from 15) and idles smooth. I don't know if the oil change had anything to do with the mpg increase. I will go back to Delo on this next change just because it is readily available again and only $0.50 more than ST 15w-40 per gallon. I think it's probably a good oil and wouldn't hurt anything. If my original oil was unavailable, I would run it with no concerns. I checked it yesterday after 2k miles and I can see through it and it is super clean. So, take it for what it's worth.
On a side note, does anyone know who makes this oil? I am sure it isn't an entity unto itself. It's gotta be Chevron/Mobil/Rotella generic or something like that.