Unfortunately there's no product that'll really do what you want here. You can certainly add protection but everything will change some aspect of the current appearance.
A good oiling (with periodic maintenance each year) will add protection, it won't stop further degradation but it will limit it. However it will also significantly alter the colouration as both boiled linseed oil and tung oil (the two main oils used for exterior applications) are some sort of amber colour and impart a noticeable yellowness once a few coats have been applied.
A good exterior varnish like spar varnish is also noticeably yellow. Also very glossy.
There are clear finishes that would provide very good weather protection, including notably epoxy finishes intended for marine applications, but they require a good film to be applied to work well and hence are also very glossy. This gloss can be reduced somewhat by rubbing down, but they'll still feel like they're coated.
Also, there's no absolute guarantee a film finish (varnish, lacquer or epoxy) will adhere properly to the wood in this condition. Standard advice for finishing is always to apply to a sound, clean surface, which these obviously don't have.