can only hazard a guess

try overrunning down a long hill, and give it a boot at the bottom, if it puffs out blue its likely to be worn guides or stem seals

if it really bothers you and you're using shit loads of oil, then i would start with the cheapest, like a rebuild on the turbo, circa £200 (assuming only seals and a balance)

then i would start looking at worn oil rings, but these dont show on a comp test, so if you're gonna pull it apart you might as well give it a refresh

Cheers, Matt