Tom ,
I don't know hold old your unit is , but it probably is'nt too much different from mine ....
That threaded slotted rod is NOT your fill .
The fill is a little bent black dipstick that sits on top of the reservior box (away from the electrical workings etc) it looks like a little black button.
Pick it up with your fingernail and lift it out (remember it is bent , not straight so it wont come out in a straight line).
If you are out of trim oil you should look around a bit to see if you have a leak or other problem.
Steve
Just because something's old doesn't mean you throw it away !