Like Denny, I thought I'd do the best for my former ride (46 Dodge) and use synthetic in the tranny. It had always shifted fine, but with the synthetic I noticed that after coming to a full stop I could not immediately downshift from 2nd to first or else it clashed. It had never done that before. If I waited 3-5 secs it was fine. I guessed at the time that the gears were spinning easier with the synthetic and tool longer to come to rest.