The oil is your answer as it is crap.
It does not hold it's viscosity very well at all.
Try changing to a decent oil like SILKOLENE PRO S, your engine will also get a lot quieter in the process.
If it has any amount of miles on it the head will need work such as stem seals as a minimum but any much over 60,000 guides/ex valves are usually shot as well.
Or it could be turbo wear, it does from your desecription sound more likely, look in the inlet pipe between turbo intercooler for oil.
tabetha