When I bought my car last Oct., it had a K&N air filter. I however took it out and replaced it with a MANN filter. For those who use them, how do you know when it's time to clean and re-oil them? I know that K&N filters work best when they start to get dirty, but at what point is it too dirty? I like sticking with OEM paper elements siince you can see when they are too dirty and replace them.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member