
I don't know how much problems some GSD's can have with certain foods when compared to other breeds in present times but in comparison with other breeds we had years ago, the GSD's I have do not have the greatest tolerance to certain foods.
We tried Oscars, years ago and both of them had the runs day in day out. Couldn't tolerant it at all. Too much cooked meats etc will also give them the runs. And when I say too much I mean anything more than a few mouthfuls per day. And that's both of them.
Mine did very well on Eukanuba LB for years, I put them on Burns on the advice of a Holistic vet but my dog had sloppy poos constantly that were just not firming up, and he was getting the correct amount by body weight. He also lost weight and he was very lean anyhow so I've changed again. He's on Hills JD now and doing very well on it with nice firm poos. Not the diet I'd have chosen for him but it suits him very well and that's the main thing. He's 10 now.
I guess the best food for an 11 year old GSD is the food that agrees with them, plus the supplements that Tohme has mentioned.