Trying different formula?

Ben's girl

Has anyone tried a different type of formula without running it by the pedi?  We took our 6 week old son to the doctor on Monday because he was grunting ALOT and having somewhat difficult feelings. The pedi said the grunting is normal as he is trying to push the gas out. Since then though he has trouble eating after about 2 ounces when he usually eats 3-4. He's on similac advance now - has been since day one when we had to supplement due to being born 4 weeks early. Thinking about trying similac for gas/fussiness. What are your thoughts?