[Daniel]

Hi Don,
You need to be careful with this one because no dirt at all must get inside the unit when you undo the top nut and that nut which is the filler cap is some light alloy and all 4 of them on my 400 were oxidised and hard to shift-you have to do the front ones too- the light alloy was easy to burr so apply wd40 and give it lots of time to work and use the right socket or a ring spanner-can’t remember size but can check tomorrow if no one posts it in meantime. Next time it’s an easy job and probably best to check levels twice a year because they can leak.
Finally Millers Oils do a ‘shock absorber oil light’ which is the right viscosity. You need to leave a small air gap between the oil level and the bottom of filler neck- best check manual for correct advice but from memory 1/4”
Stay safe (and sane)
Daniel