Hard to pick a “first”. I’d say the neutral clutch Johnson TN’s would be in the running, but there were others about the same time. Seems to be somewhat related to gearshifts and stainless steel shafts.
Interesting question.
I read something awhile back, some were using john deere cornhead grease for their lower units, not having any problems using it, also much cheaper on price.
The TN’s predecessor, the direct drive TD-20, had a recommendation of 90W in the service manual. Some of the TD-20 models have “GREASE” stamped above the gearcase filler screw. Not sure if those were left over from the pre-war TD-15 model.