If you need to change the o ring at the lever end you need to make up a tool with two prongs (I think) 16mm apart to undo the part that holds that assembly in. I made mine by grinding down an old socket but someone on here recommended doing it with one of those flat hole borers for timber. I.e. grinding that down so it has two points 16mm apart.
The biggest discovery I made was that if you take the red tube from a can of GT85 and push it into the nozzle of a plastic syringe you can squirt the oil back in carefully without spilling it all over the place and can get the post properly filled without air bubbles stopping it going in. Although the instructions state a specific volume of oil I found you just need to fill it right up until you can’t get any more in.