I use corn head grease.I am not kidding,that is what it is called and it is available at John Deere.I ran 90wt until it started leaking,switched to this stuff on recommend by a farmer friend.No leaks,trans runs cool.It is thin enough to drool down into the transmission as you put it in.I used a plastic syringe,but you could just as easily use a grease gun,it comes in a tube for that application.
Of course,I never recommend any of the things I do.
The stuff is made for gear boxes,actually.Corn heads,whatever the hell that is.Agricultural terminology