You are suffering from the detorats!!!! Check your rear crank case vent your probably blowing oil out it too.
Easily fixed by changing out the bad piston or pistons. That's where your boost is going by and pressurizing the lower crank case!!!! You can scope the pistons to verify or run a compression test. In the picture below the almost white section on the pison edge is the top compression ring.
The only other thing it could be is your rear crank case vent is soaked with oil to the pressure blows the oil cap off. I suggest you try the scope or at least a compression check when you do the compression test make surre all the plugs in the other two cylinders are out.