As title. Had mine in for 18 months and not cleaned it yet. Is that bad?

It's not too bad at all.

The service interval for the oem paper one is 4 years, and the pipercross can hold more dust.

If it looks dirty, give it a clean, or bang a new one in.

Cool, thanks

Yep Soapy water works well on foam filters just oil it after you've washed it and dried it

Been using K&N for ages on various cars, clean them every couple of years with K&N cleaner, usually use hoover to give it quick clean 4/6months in between, just don't over do the oiling after cleaning.