Oil viscosity 60 series 12.7

[2dlb]

I have a 12.7 60 series Ddec Iv with a recent o/h. We installed a full tilt manifold hopefully to get passed the cracking problem with the stock ones that seam to break annually and a 171702 bw turbo. Calibration was done by a reputable tuner. Since then every oil change at 15000 fails drastically the vi index of 15w40 gets to 80- off the scale. Oxidation and nitration are high as well. Does anyone have any thoughts on this. Is the 1702 to big for this set up.