I have tried Friso Gold 1 (when baby was 1 month, just one supplement to breast milk), then switched to Similac near 2 mths, as he seemed no comfortable pooing. Yes, Similac bubbled a lot somehow... esply if using Avent milk bottle. He was pooing fine (but Similac was just 2-3 times supplements for breast milk... he was drinking 5-6 oz per feed for both breast milk and Similac then, but he didn't wake for nite feed). Then later, just before he hit 3 mths, he started to refuse to drink Similac. I dont't know if it is the taste or he did not feel good about drinking it (my cousin, who has 2 babies, reminded me that sometimes, baby may know their own body system best. baby poo-ed normally, but maybe there was wind or something.. he seemed to fart very often then). So I swtiched to Nan 1. I love the consistency- ez to mix, and light like breast milk. But after 1wk plus when we were just into the 2nd small tin, he refused to drink again. Then we switched Mamex Gold.
He is turning 6 mths. Still on Mamex Gold, but we already weaned him on some cereals since 4 mths, as he was losing interest in milk (again). Paede gave us the go ahead to start him on solid then. So far Mamex Gold seems ok for him.. no poo-poo problem. (I put the FM in his cereals as well) Mamex Gold contains pre-biotics that is supposed to help good bacteria growth in baby's gut. This can protect baby's stomach (from harmful bacteria etc) and prevents constipation.
Nan contains pro-biotics, which I think is the good bacteria culture itself in the milk. This also promotes healthy gut, and prevent constipation.
In the market, so far I think only Nestle Nan and Mamex Gold (or the Mamil Gold series) contain pro-biotics or pre-biotics.