before you start taking anything off; you can rule out some things

things it could be:

turbo seals
valve stem seals
valve guide
piston rings

is the engine a new build or is it old? mileage?

a good way to see if its stem seals/ guides is to overrun down a big hill; then at the bottom give it a boot - if a big puff of blue smoke appears, then "more tha likely" its likely to be guides or stem seals

you can simply get a compression test done for piston rings - any garage worth there onions will have this equipment

start with that then come back