Pennzoil HM seems to work great in my Firebird. It had a slow leak that Mobil 1 HM failed to make a dent in, even after 3,000 miles. After ~1,000 miles of Pennzoil HM, it seems to be slowing down at last. In terms of performance, sound, etc. I can't notice a difference. I haven't tried their Super HM, however, as I can't seem to find it here at any place aside from Walmart for a reasonable price.
It takes time to stop leaks with HM oils, the M1 probably was working on it already and the HM additives in the Pennzoil finished it up.