I know for certain that "H" was the aircraft grade because I own a gargoyle "H" grade one gallon can. I found it when I was 12yrs old at the Bennington, VT antique car show and I still have as of yet to meet anyone else that owns or even knows of one(that would make an interesting thread on its own). References to the gargoyle "h" grade were also made in several issues of the "gargoyle" vacuum oil new letter which I have packed away somewhere and feature beautiful pictures of early aircraft.