There is a restrictor in the small boost pipe that connects to the turbo outlet nipple.
This is only 1.0mm on the PPP WRX (1.2mm on the WRX) so it can clog especially if over filled with oil.
The smaller the hole gets the more boost spike or overboost you will see.
Overboost will cause oscillation especially on motorway when trying to regulate part throttle boost in 5th
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