I got an SLP lid and a K&N filter at the same time. One of them (I think the filter) came with a sticky rubber part that goes around it to help it seal. Mine was split a little so I never put it on. Will this cause an air leak?

 

It might. Depends. Does your lid appear to be airtight along the edges? If it does 'leak', it'll just end with a little unfiltered air going into the engine.