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I don't see why they wouldn't want you to use Rotella T6. That's all I ever ran in my first 6.0, and I NEVER had injector issues (well, not with the injectors themselves - ICP/IPR is common though). My 2nd 6.0, I was coerced into running 15W40 and ended up getting stiction, so I added Archoil AR9100 and it helped, I never did switch over to T6 before I had 2 injectors crap the bed on me.
The only reason I can see them telling you NOT to use T6 (which is what most guys use, last I knew) is because they want to sell you more injectors.