I did some online investigation today into cleaning suggestions for the whitish discoloraton which often forms on rubber focusing and zoom rings on camera lenses. Exposure to sunlight and hand oils seem to contribute to oxidation of the rubber.
I read many different online cleaning suggestions using water, alcohol, dish detergent, Armorall, and 303 Aerospace Protectant. Alcohol will likely contribute to drying out and deteriorating rubber, silicone based protectant can migrate to undesirable areas of your lens that can be very difficult to remove, and Nikkor owner's manual strongly warns that water moisture on the lens will lead to rust and malfunctions.
Taking into consideration everything I read, today I tried cleaning the rubber rings on my older (2006) Nikkor lens with nothing more than a toothbrush slightly moistened with 100% strength Murphy Oil Soap (which is manufactured to safely clean wood). I gently used a small circular motion with the toothbrush against the rubber as I worked my way around the lens rings. Results were immediate and outstanding. Lastly, I thoroughly wiped the lens dry with a clean, dry, soft cloth. The rubber is again jet black, feels nourished from the oil soap, and the lens again looks new.
I think nothing could be easier, more effective, and safer.