Now is good because the tree is growing. End of winter is best because the whole growing season lies ahead.
I haven't scientifically tested the different pastes (this is on my list) - any putty, paste, glue, sealer should be fine.
I asked a proponent of the foil method in Japan what's behind it and the idea was to keep the area well sealed and cool. He used some substance that might have washed away without it and the foil both kept the area somewhat sealed and cool as it reflects some heat away from the wound.