Turbo issue?

Hello everyone. I took my Mini Countryman in due to white smoke in exhaust and slow start when accelerating. I took it to my personal mechanic who said my turbo was failing. Since my turbo is under warranty, I took it to mini dealer where they found excessive oil in inner cooler and intake boot. They recommended an inner cooler flush and a moc decarb. My question is, once they clean the oil out, whats to stop the oil from entering again?