Most folk are putting in 'JD Cornhead Grease' into most generic gearboxes these days. It's designed to sit like a grease at room temp, but with light heating it works like a 90w gear oil. This lets it lube gearboxes, but not leak out through bad seals.
Agco has a similar product.
I hear Napa and others can get it, but it's not often regularly stocked?
This time of year, any JD ag dealer will have a rack of it up front....