I have just done this job now and I took out about 230~240ml (each strut) of what I would say was 20 cSt.
So,
the dry fill was ~250ml per strut, of a relatively thin oil, 5 or 7W (maybe 10w, depends on manufacturer's rating).
However, the hex of the cap heads on my struts are damaged, so someone without the right tools has been at it already. I can only say what I took out.
It is a conventional USD fork, and the damper rod is attached to the cap (it does not spring off into the workshop, like some!
). Just spin the tube once it is loose.
I did not dismantle it but just drained and pumped it out.
My bike has 6,000 km, the oil was quite clear green with no metallic particles I could see. It was not dirty but it had some black particles in it.
The tubes slid freely on the chrome, and there are no leaks, so the seals and inner collars I guess are OK. No need to do that job, yet.
The strut cap heads are close up to the top triple, so I removed the struts first and used a strap grip to remove the caps. I guess they were around 20~30Nm. (I was not able to quite tighten back to that as they had oil on the tubes and no longer gripped so well. It might need for the handlebars to be removed if the caps are tighter or should be tightened back up more.) The caps are 24mm. A spanner with a thin head could do that job (I only have sockets in that size.)
I refilled with just over 200ml of Dexron II (35 cSt).
I did not see the level of the oil down in the tube, so the air gap is quite large. I don't think a lower level will change the behaviour much, but I put a little less in anyway as the struts are too hard for me, and I think 200ml is enough and I don't know the history of what these have had.
The stroke of the strut is very short, under 100mm, so I think it does not need much oil. There was plenty of oil for normal damping when I pumped the strut to purge the air.
I retightened the lower triple bolts to 23Nm, and upper to 16Nm. The axle pinch bolt to 23Nm and axle nut to 80Nm.
I would say the ride is very slightly better.
I think overall for my weight, the struts are overdamped, and with the spring rate does not give a good ride. Perhaps I might try 175ml of a 15 cSt oil in the future.
I think the basic issue with the strut is that the stroke is too short, and so both the damping and spring rate has to be very firm anyway. I guess it's better on German or Chinese city roads, I am sure, but on UK roads it is a constantly bad ride. I have some alternative struts ready for me to try, perhaps next year.
Hope that helps!
