I have two shelties and feed them sardines in oil twice a week.  It is good to hear that the cheapies work just as well as the John West varieties.  I had been feeding mine John West as they have more Omega 3, DHA etc.  I am a serial label analyser.  I bought Brunswick today as it has 89% sardines where the other brands have around the 70%.  I found that the varieties with more oil have more Omega 3s etc.  Tuna isn't as oily a fish as sardines and therefore doesn't have as much of a benefit as sardines.  Sardines from my label studies have shown to have the most Omega 3s in them. 
Regards 
BarBell