I have mentioned things to people at traffic lights. I tend to nod at them and say hello and see how they react to that before I offer any advice.
As long as i am friendly and look like i know what i am talking about, I have found that people have been pleased to have been told.. Talking to someone when they are riding I reserve for serious problems, as I have found that to be more problematic. Not really sure why, but when stopped people seem happier to chat.
I have adjusted some really bad saddles (down as well as up) pumped up a few tyres, but never oiled anyone's chain, suggested it mind..