I put Mobile1 synthetic in my Geo Metro and had a 1qt puddle under my car every day after work until I changed back to Dino. Yes, it leaked oil anyway, but not a 1qt puddle while shut off every day for a month until I changed back, when it went back to 1qt between changes, no puddle. And no, the drain plug was not loose, but I did check it. It had one o'them white plastic gaskets on it (forget now what that's called), which seemed fine. And I did nothing but change the oil to synth and back. That was in ~1992, so there are more additives in synth, and more advances in seals since then, I'm sure. I don't know from where it was leaking.
