My brother had a turbo truck and went to the track often. One time he ran it started to smoke and power dropped off. We pulled the heads and replaced the gasket wverything looked ok. But it rattle and ran bad and stilll smoked smoked so we pulled the motor since we did take the rod out we didnt realize at first the all four pistons were down. And tunring the crank did not bring them up to the top any more. Turns out he found the tq limit of his factory rods. Bent all four of them at once. Once we puled it all the way down everything was broken bent or burnt. It was amazing how bad it was and still ran ok. It broke off the line and he still made a low 13 second pass. It still made boost and pulled when you drove it. But it was way down on power.
That does not relate to your issue just a problem i had in the past and didnt catch the first time thru. You must be missing something fuel or spark or compression has to be off. Unless the valve seals are just flooding the combustion chambers with oil but i havent seen any thing like that in years. But sitting can cause seals and parts to stick and fail. I can see the cam sticking and stretching the chain a little. But cant see what else will cause actual detonation to occur all of a sudden.
If you were close i would offer to come look at it. LOL