What kind of oil or grease should be applied inside the shock tower to prevent it from rusting? Can you just spray it up in there or what? Suggestions much appreciated.

Just so long as bare metal isn't showing anywhere, you should be all right. Undercoating is good.

More importantly is the spring support. The little holes in it can be plugged by debris and road garbage. Stones and dirt collect inside of it. Wash that thing out, since it's more likely to be problematic than is a corroded strut tower, especially on the last generation 240.

Keep your eye on the seams inside the engine compartment for any signs of rust (strut tower seams).

Unless you're running through salt water I would'nt worry. If you pop the plastic caps off the tops of the strut tower with a screw driver you could lube the mount to strut area. Don't use anything that would harm rubber. I have never heard of this being a concern,but climate and conditions may call for it.