Reading lubrication section for 8N tractor, found a lot of grease zerks not accepting lubrication due to the joints themselves being plugged up with old grease/residue.
Perusing tractor repair forums & watching DIY videos, found a pretty simple, effective solution:
Mix up homebrew penetrating fluid using oil, transmission fluid, gasoline, acetone (various recipes, see video link), use tool made to inject penetrating fluid under pressure into grease zerk, dissolving and removing old congealed, hardened grease, then Lube with regular grease of your choice (opinions vary, see https://www.youtube.com/watch?v=mMmSQSjraSE).
These tools are really quite nifty, and if you look around enough online, you'll find some for around $8 with shipping, but if your tool is like the one I wound up with, you're going to have to do some modifications to make it work. Tool came without an end which would fit on grease zerk, so had to make an adapter from hardware store ingredients (still cheaper than purchasing major name brand tool).
For adapter, used bolt that would screw into tool, that would also screw into grease gun zerk coupler, & drilled hole through bolt for grease passage then red Loctited homebrew adapter into grease gun coupler (necessary to obtain tight enough fitment).
Here is video of one type of these tools, clearing out plugged grease joint: