Ah, Kemosabe, you ask intelligent questions. The answer is a little more complicated.
I suspect it depends on what type of resin you are using; pine, spruce, fir,... I have mixed whatever is available together, heated, then strained/squeezed through cheesecloth to get rid of bugs, sticks, needles. Pour into a large coffee can (about the amount needed to gum one modest sized canoe) for later use, or into a frying pan (I pillage them from neighbourhood recycling blue boxes, even though they shouldn't likely be there).
If cool and hard in a coffee can, reheat in a large frying pan and add 1 heaping tablespoon of 100% vegetable Crisco (this is what I was taught to use by 2 native builders), and stir well. I think it may take some experimentation to get the proper consistency. Let cool enough that it is still pliable enough that you can apply it with a stick.
In the film, Cesar's Bark Canoe, the "gumming" process begins at 46:40.
They don't show what or how much "fat" is added, but the consistency is pliable enough to work into a seam using a thumb.
I recall Steve Cayard favours peanut oil. Or was that Henri Vaillencourt?