FWIW. Its my understanding that the only real benefit from using gold (plated) anything is to allow more efficient conductance of current AND to make this effective both surfaces should be gold. So if your speaker connectors are not gold plated its not a 'deal'.
Your bananas are gold plated and what you are probably seeing is a wearing off of the gold plate from use. It could be a build up of gunk but I doubt it.
Also, consider, by adding any lubricants to your connectors you may be inviting the accumulation of gunk.
Personally I focus on keeping the connections as tight and clean as possible. If you are not already doing so consider using locking banana plugs.
FWIW.
Your bananas are gold plated and what you are probably seeing is a wearing off of the gold plate from use. It could be a build up of gunk but I doubt it.
Also, consider, by adding any lubricants to your connectors you may be inviting the accumulation of gunk.
Personally I focus on keeping the connections as tight and clean as possible. If you are not already doing so consider using locking banana plugs.
FWIW.