This is an old topic -- just adding some info in case anyone else is having trouble.
The 8N manual calls for "heavy duty motor oil - SAE 50 HD, or straight mineral gear oil - SAE 90" in the steering gear case. But in an old 8N, this typically leaks out. On another forum I found suggestions to use a very soft grease instead of oil. John Deere Corn Head Grease was specifically recommeded. I've been using this for 10 years, and it seems to work quite well -- it still leaks out, but more slowly.
Now, getting the grease into the steering gear case can be an interesting challenge, especially because there were at least 3 different places to insert lubricant, depending on the serial number of your 8N, with yet another arrangement on the 9N/2N. For 8N ser no 216988 through 452912, you remove a bolt from the right side of the case. It's directly behind the proofmeter cable, so you have to take off the proofmeter cable first, then find the bolt with an 11/16" hex head (you'll probably need an extension on your socket wrench). Take this bolt out (don't lose the lock washer!), and insert lubricant through the hole.
On later 8Ns, there's supposed to be a 1/4"-20x5/8" screw on the steering column that's used as a filler plug. Earlier models have a different steering gear, and I'm not sure where the filler plug is.
To get the John Deere Corn Head Grease into the filler hole, I use a grease gun, with the zerk fitting removed and replaced by a little plastic nozzle I scavenged from a package of silicone seal. Messy, but it works.