Hmmm, tricky. I haven't seen that before but most of my time was with 7.3ls. Typically when letting of the accelerator the PCM would "close" the vains in the VGT charger which might cause that spike but it seems unlikely because of the time it happens. 1. usually the VGT would be much slower to respond than the energy lost when fuel is reduced. 2. usually the EBP pressure would prevent the vains from closing too slowly.

So

1. I could be wrong about the throttle body, but I'm fairly confident that's not the case

2. I wonder if the compound configuration might cause a pulse if the VTG is closing fast enough, and/or the low pressure charger's wastegate closes too quickly.

Also the low pressure charger should be wastegated iirc which usually helps prevent that

Second. It's normal to see oil in your charge pipes because if the crank case vent. Not a lot but more than you'd expect, just throwing that out there. You may want to wiggle shafts to make sure they don't have any play in them if you havent already.