The many answers have been SO helpful , and directly answer my questions.
I have printed all of the replies and will keep them for my notes.

I guess the 50 yr shingle and using WeatherGuard is indeed overkill. But,
I have had this shed for 20 yrs , and it looks nearly as good as the day I
had it built. I plan to live another 39 years, so I just want to keep it in
# 1 shape. The cost difference for the entire roof would likely be less
than $100.

Thanks again for the real nice replies, and I welcome any other comments !!!

--James--