Whenever I have bought a new sonar, my first move has always been to grease the contacts on the plugs an sockets. I use a silicone grease formulated specifically for the purpose. It's a Dow Corning product called MS4 Silicone Compound, though I see that it's now been renamed DC4. I've used it on on all electrical contacts on my boats.
Corrosion block grease will work very effectively too. If you only have the oil, then repeat the application a bit more often.