FYI, this topic comes up at least every six months or so on the message boards, and debate often rages. Detroit's got more experience with these engines than anybody else, why is it that folk don't follow their recommendations?
Here's a link that you should print out and keep in your engine's maintenance manual, direct from "the horse's mouth":http://www.detroitdiesel.com/support/on-highway/manuals/Lubricants_Fuels_Coolants/7se270.pdf
The section that pertains to the two-strokes is in section 2-2.
In a nutshell, you've been using the wrong oil, not an uncommon thing. Especially since straight 40wt is not usually on the shelf at Wally World, it must be ordered.
Chevron Delo 100 and Shell Rotella T are two of the more common brands used, altho there are lots of others. Easiest is to order from your local oil or petroleum distributor, located in your yellow pages. Comes in gallon jugs, six jugs per case. You'll need two cases, because the 8v71 takes seven gallons plus a smidgen more with a new filter. You can also order it in five gallon pails and 55-gallon drums.
FWIW & HTH. . .