Fork oil

I have found that 190 cc of a good medium grade in each leg works best for me.

Your choice. (Mine a 76 Mk III)

Depends on personal preference and clearances of damper valve to damper tube. So anywhere from 20W to 40W

No 20W fork oil to 40W fork oil. I did try 20-40W once but it acted like 20W fork oil.

I have found that 20W Bel-Ray fork oil suits me/my Commando best (can't remember the quantity). There was a lot of talk some time back on the Access Norton site when it was concluded that the same grade in different makes felt different ie you needed to specify the make as well as the grade. I seem to remember that Mick Hemmings recommended Bel-Ray (and one other which I can't remember), so that's good enough for me!

fork oils of same apparent ‘weight’ are NOT the same in fork performance, hence the importance of good personal recommendations. ( why it has developed like this....)

     This has been empirically thoroughly tested by others.

Exactly my point.

As Jan Nelder has pointed out, manufacturer's fork oil grades are only comparable within that maker's range. A detailed comparison of a number of ranges is here, including objective measures in centiStokes at 40º and 100º.

Manufacturers' data sheets for any oil should offer cSt data, so it is possible to compare the viscosity of fork oils with that of engine oils. Of course there are other reasons for possibly preferring purpose-made fork oils to engine oil.

Just curious,  what is the purpose of increasing the amount from the 150cc recommended in the riders manual?

Depends on how you ride also the weight of rider and if carrying a pillion on a regular basis would influence the ''weight'' of the fork oil.

Manufacturers recommendations are based on an average rider, riding at average speeds

I use 10W-30 tractor universal oil. Not only is it a diesel engine oil, it is also a hydraulic oil and is used for oil-immersed brakes. Makes it ideal for both forks and primary chaincases. And tractors. And it's cheap.

As well as Mick Hemmings BelRay 20W recommendation Richard mentioned, Norman White has a very useful fork section in his recent book and also recommends a 20W fork oil, Morris in this case. They do the standard or Race type which is the high performance version but either would be OK for a standard bike with forks in good condition.

Interestingly he only mentions the standard 150cc changed every 6000 miles which is a good starting point but others here have had success with 170cc or more. The extra oil covers the damper and also decreases the air gap in the forks, a common fork tuning method and may account for the various different setups preferred here.

I measure the specified oil level in each leg with the springs removed and fully compressed rather than the cc method. My modified Roadholders are 150mm and Yamaha 169mm at the fork centreline from the top of the stanchion to the oil with the bike on the centre stand.

The higher volume actually comes from the production racer specification sheets from the early 70's (not the service release) which I believe have been linked to in the past on the NOC website. The 150cc is mentioned in the Domi service instructions, this makes sense as the damper tune is nearly two inches shorter than a Commando damper tube. The damper tube needs to be covered in oil. You would seriously need to reduce the air volume in the Commando fork for it to be noticeable in use. Covering the damper fully is way more important than the reduction in air volume. 

170cc of 15w fork oil. I have a sectioned slider here that I might do some measurements with. 

Well Les started something with his simple request for guidance on fork oil. The quantity  ranges from 150cc to 190cc with pretty well everything in between. The weight goes from 20 to 40  and varies from fork oil to tractor oil with engine oil in between.

If anyone is still interested, I use 170cc of Bel-Ray 20wt fork oil.

I will be interested in what Ashley's measurements tell us.

Considering the leakiness of the top nut the effect of reducing the air volume is likely near to zero. Take a complete fork less spring but with oil and mount in a vice, as you move the stanchion up and down you can hear the hissing at the top fork nut where it screws into the stanchion. 

Riding along in the wet think about all the moisture laden air pumped in and out and mentally prepare for the next fork oil change if you have internal fork springs. 

Never had that problem on my Atlas which has Commando disc forks and did over 100,000 miles. There was a thin paper gasket under the stainless top nut washer but can,t recall if it was ever a Norton part. Modern forks I,ve seen have O rings.

A leak like you describe will pump oil as well as air and be very noticeable. I found that out when I tried an air equalising line between the forks like air forks, with drillings into the top of the stanchions. Didn,t quite seal it right and a weep of oil resulted even with 150cc of oil. I soon blocked those up and stayed with the measurement method for equal levels.

I drained one of the old forks in the weekend and the used oil which has been sitting there since 1988 and had quite a few winter miles on it, was still clean with no signs whatsoever of moisture emuslification. Pretty sure the oil was Bel Ray 20w which was all I used in the 80,s.