Most off-the-shelf products will just add some temporary depth/shine back in. Nothing wrong with that, but as the product dissolves/evaporates/melts, the dull color returns.
There are some actual "dye" products that will re-stain it a deep black, such as Forever Black. We used it on my brother's G20 years ago and it did look a lot better afterwards.
My advice would be to re-stain it with one of those dyes, then stay on top of the protection by spraying it with a UV protectant periodically. 303 Aerospace Protectant is quite durable and targeted specifically at this use. I've had fantastic results with it for many years.