Hey Peter, got that sign from my Father's house. It has been sitting outside there for 8 to 10 years with no protection. It is showing signs of the start of degradation and has green moss growing on the back side. What should I do to clean / protect it?
oh boy.
I have to say that my personal take on such things is to let nature take her course. It's kind of either a losing battle with nature's graceful touch, or a losing battle with the hapless markings of Sisyphean human intervention. You've got a lot of end grain going on there: on the ends, of course, and on the cheeks of the relief cuts where the main flat surface is established, and then on the sides/edges of every thru-cut forming the negative space and the sides of every letter. So, it's like a giant wicking/checking/cracking thing you got going on there. Cool tho.
Anyway, I would only clean with scraping and dry scrubbing (stiff plastic bristle brush), and perhaps sanding (if you actually want to remove the "patina"/oxidation).
Trying to preserve it is problematic because most preservatives will end up looking blotchy over time (shiny/dull areas and oxidized/non areas). However, the first step in preservation would be to wick as much penetrating oil in there as it'll drink. I would be using SeaFin primarily because I'm familiar with the stuff and don't know of many alternatives. (I'm sure there are plenty of other viable products out there.) SeaFin is non-viscous enough to really soak deep into the endgrain; but is very effective, when cured out, as a hardening/preserving agent. (things like polyurethane or epoxy may work; but are basically plastics, which will perhaps not wick nearly as well, and will exacerbate the blotchiness issues in spades.) I would liberally dab with a 2" paint brush all exposed endgrain (log ends to letter edges) until it finally stops drinking it up. (this, of course, I would do after at least a week or three of hot, dry weather.) IF you start in on the other surfaces, the "flats" (anything that isn't endgrain), you'll be heading into blotchy-ville and shiny/dull patch city. I wouldn't be going there, or if I did, it would be just a throw-away "evening-out" coat to mitigate any wicking slop-over from your endgrain exercise. I would simply brush on the oil one time and wipe away any excess after 10-15 minutes (so as not to leave any surface build → which is what will result in blotchiness after a year or less of weathering).
The wicking into the endgrain (especially on the letters) may buy you a number of years where they don't start to crack and disintegrate. Conversely, treating ("painting") primarily the flat/long-grain surfaces for a renewing effect (but not addressing the end-grain wicking properties) will surely lead to an annual chore with sketchy results. It would perhaps give it a face-lift while everything under the skin deteriorates and cracks.
Seriously tho, unless you're just looking for 20-30 hrs of work this year, and annual maintenance (because I know for a fact that you're not busy enough as it is), I would strongly suggest letting this one go and enjoying the natural wonder of graceful decomposition.

I think it looks pretty the way it is (with the gray oxidation and the moss n' stuff).