How do I clean out oil to prevent dieseling?

I got my first air rifle, the Hatsan 95, but it was really loud the first time I shot it. I noticed some dieseling from the smoke that appeared, and am wondering where to clean the gun. Do I clean the oil in the barrel only? Do I have to take apart the gun and remove the oil in the compression chamber?

Also, I'm using .177 pellets at 7.4 grains, and I read that might be causing the pellets to cause supersonic booms. What grain pellet should I get to be subsonic?

Finally, do you think the loud sound from the Hatsan 95 is from the sonic boom or from dieseling? There is only smoke after I fire, no flash.
 
Buy cheap pellets if you don't want to wast good ones. My Gamo Whisper Fusion Pro or Elite or what ever they call it did the same thing. I shot it I'm my back yard and it sounded like a .22 rimfire going off scared the crap out of me as I like in a city and it was LOUD. I went in to my basement and put about 60 rounds through it to break it in. I shot it in to an old book.
 
As We all know dieseling is caused by using petroleum based oil .that ignites behind the pellet when the Gun is discharged.This increases the pressure behind the pellet and increases the heat in the gun .This can cause sealing o rings to fail and in the worse case cause the pellet to deform possibly damaging the barrel or breaking a spring .or even harming the shooter or persons near by. .