Bill, You are a treasure! I look for your common sense replies to just about everything! However, We used to have discussions like this over at "Yesterday's Tractors". Only there it was GL1, GL2, GL3, GL4, and GL5. Which one should I use? GL1 was the stuff Henry used in the N series tractors, but it was so obsolete it was hard to get. The higher numbers contained sulphur that would eat up certain bearings in the rear. Never did get a, one GL fits all, answer. Fortunately, I don't think there's any yellow metals in a model A trans. Have a good one!
Terry