TonyV, I have an Advanced Jumbo, it wears & loves D'Addario J-17s (Mediums/13s). They work for me. It is certainly designed to take 'em.
I had a mid-'70s F30, played D'Ad lights/12s on it. I use the same on my Gibson LG-1 & LG-2. I'm not sure what the F-20 is, but if it's regular "small bout" size like the above, I'd figure it could handle lights just fine if it's structurally sound. My little Gibsons are a '53 & a '62.