My understanding on not being able to run bio in a 6.4 has to do with the DPF. I believe, and I'm sure someone can correct me if I'm not totally accurate, that during post combustion a small amount of fuel is injected in the combustion chamber so that further burn occurs as it enters the exhaust system. Cleaner exhaust is the desired result.

The problem with biodiesel in this system is that it doesn't completely combust and much of it winds up washing down past the piston rings and into the crankase diluting the oil.

I make my own bio and am currently running B90+ in my 7.3 and I see about 8-10% reduction in mileage and power.(Correlating BTU's as compared to #2) But I figure so what? It has phenomenal lubricity, I'm using a free waste product to make it ( that 'off the grid' feeling), and my truck's plenty powerful enough as it is for my needs.