80/90 would be OK if the seals are perfect. It has been my experience that most of the seals are NOT perfect, so the 80/90 tends to make a mess.
I use John Deere Corn Head grease in my manual boxes. Recently someone else recommended a specific steering box lube. If you do a search on steering box grease you should come up with the discussion, which was either last week or the week before.
