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The only way to get the oil out properly is to disassemble and clean with cloth. While it is apart you can inspect and relubricate the spring and piston. Try to avoid getting lube or oil ahead of the piston or it will diesel and damage the piston seal or even break your spring.
If you are getting only moderate dieseling (smoke) you can shoot it out and the rifle will get back to normal over time. If the rifle is exhibiting serious detonation I don't know of any solution outside of disassembly.