For the past year I've been running Pennzoil air cooled at 50-1, there seems to be a lot of residue after just one flight. I've used Amsoil at 64-1 since 1985 and loved the performance and little mess it makes. I feel I'm not getting the performance from Pennz that I did with Amsoil, but I don't know if I can trust the new formula that came into play about 2 years ago. Wish they would have left well enough alone.