I've used brown sugar and added it at the last 15 minutes or so. What I found is that it adds rum like notes to the beer. I used 4 ounces and, according to my notes from Xinghu (spelling?), it was excellent, coffee type notes, along with the rum characteristics. I'd use it again. I've never used treacle, but tasted a beer with it and didn't care for it. I found that molasses didn't add the same flavor profile as brown sugar did. Don't forget, there are different 'colors' of brown sugar. Use the darker one for darker beers.