Recently I had the odd running rough and cylinder 1 misfire cel recently, it would come at go at will. Initially, it only started smoothly after I changed the plugs. I then had no issues for several weeks. It happened again on new years day during a run when driving around 50mph. It has never done it under heavy acceleration. Sunday I took off and cleaned air filter, pcv and turbo inlet pipes. No oil at pcv on hose but they was at the turbo inlet. This would explain the recent blue puffs of exhaust. All was cleaned and car ran normal today although I never pushed it. I had it booked into the local GMC dealer today. They have just rung me and said No1 cylinder has NO compression at all and that it needs a new engine as the rings are shot and there's scoring. So, if a four pot loses one cylinder, wouldn't it have problems starting, let alone driving smoothly, which it does and will do. Only No1 plug was oily when I changed them which did suggest an issue around there but come on, am I missing something here?
The car is staying there overnight. Oh, and they expect it to cost $6k to replace with a GMC warrantied engine.
Any comments/advice appreciated

Thanks,
Chris