gps

Hi guys,
I drive a 2006 E280 CDI 4-matic (mieage 170,000 kms).
Last week I did 2,000 kms in one stretch. About 700 kms from home I checked the oil level, which was ok. However, I also took off the cap of the motor and forgot to put it back on again, causing the engine to spit out a lot of oil on the last 700 km stretch.
The oil loss was quite severe as I had to fill in 8 liters of oil before normal oil level was reached (manual says car contains 10,5 liter oil).
After this incident the car started to make a whistling sound which I assumed was the turbo which was damaged. At the garage they checked the car and informed me that turbo is ok but cat and particulate filter needs to be changed.
Engine runs perfect otherwise.
I am a bit confused as to the cat problem as I think the cat would only block if oil comes out into the exhaust from the turbo or due to bad tuning.
Engine runs good (it appears) and turbo is ok.
And why does the particulate filter need to be changed? It should be self-regenerating.
Can it be only a coincidence that the cat gets blocked after this incident?
Any good advice will be appreciated! Thanks.