Like a lot of other Astra owners on here I fitted a K&N filter a couple of years ago. However I've recently bought a BMC CDA filter for when I fit my Dbilas inlet manifold shortly. I dismanted the BMC to clean the filter, it looks exactly like a K&N inside BTW, I decided to buy a K&N cleaning kit from Online Autosport as I thought it could do with a clean. Whilst doing the BMC filter I thought I may as well do the K&N panel filter. Removed it and damn it was dirty, knocked all the crud off the bottom I could, sprayed it with the cleaner and left it for 10 mins. Hosed the filter down and allowed it to dry, then sprayed it with the oil in the kit. Waited 20 mins and checked for any missed bits and resprayed the filter again.
Put the filter back in and the difference is very marked, first thing I noticed was the tickover, normally its at 750 but now its dropped to 700, the pickup is cleaner, the response is better, everything just feels just so much better.
So if your like me and had the filter in for a while can I advise you buy a cleaning kit and have a bash. I'm sure you won't be dissapointed.