Salmon Fisher is right on. You can lube them and try to loosen them up but any progress you make won't last very long at all. It can look pretty intimidating to replace them but it isn't as bad as it seems. Even the adjustment of the new ones is not too difficult. The length is important so if you can't determine the right specs just take them out and measure. Most marine stores will have the cables in stock or can get them quickly.
One more thing to look at is in the controller itself. There may be tension adjustments for the handles and those are usually easy to check even if it is wishful thinking.