Agree with all.
That old grease turns into concrete. Get that old stuff off if it really bothers you but I would not sweat it too much. Get it as clean as you can, put on the new lube and call it a day. I like the silicone spray.
While the brackets are out: look for any chow marks that look like it's cutting into the metal. As long as they are straight the lube does not do much. If there's signs of grinding ... that needs to be resolved.
faz