Belt drives and oil contamination of the clutch.

I'd be interested in answers to those questions too.

The standard bronze clutch on my Mk3 slipped badly at about 5,000 rpm - dangerous if passing a line of cars in 3rd. Cleaning helped for a few miles but it kept coming back. I've fitted the RGM pushrod seal and changed to Surflex fibre plates and changed from a semi-synth car oil to a 20/50 4T specified for wet clutches. Seems to have worked. But having changed three things I don't know which has had most effect.

I would note:

• Clutch slip was never a problem on any of my Commandos (including Mk3) when they were in daily use back-in-the-day(TM). Then I was using Castol EP90 in the gearbox and and a mineral 20/50 in the primary; now I use a 75/90 synthetic in the gearbox.
• Running Surflex fibre plates dry in belt-drive race bikes, I never saw any oil contamination of the plates. But those bikes were parked on track stands so they never spent much time leaning over to the left.

So is it that modern oils are much "slippier" or infrequent use, with the bike parked on the side-stand for days/weeks, allows the oil (primary or gearbox) to permeate the clutch where previously it was flung off? Or something else?

The subject has been covered extensively over many years:
https://www.nortonownersclub.org/support/technical-support-commando/clutch

and, of course, Mr Leadbeter's exposition on the shortcomings of the clutch itself:
https://www.nortonownersclub.org/comment/16032#comment-16032

But, having read these articles, I'm still trying to understand why I never, ever suffered from clutch slip when using my Commando regularly in the 70s. And the several Commando owners I knew never complained about their clutches either. Yet now, using the exact same parts, it can be truly awful. Yes, the parts are half a century old but they've just been in a box on the shelf for 40 years.

Things that are different are: oils and time spent parked. Is that it?

To answer the question on the Gearbox oil, it usually contains extreme pressure additives that could affect the clutch plates more than other oils.
I don't use the side stand and don't get gearbox oil in the in the clutch. 
My clutch used to slip but on the first aggressive overtake of the day, after that it would be fine all day.
If you fit Barnett plates then they should work fine in oil as they are designed to run in oil but will also run dry. 
Most race bikes with belt drives most probably don't experience this much as they are usually on paddock stands and thus upright. 

For gearbox oil to get sucked along the box mainshaft and into the Commando primary case, there has to be a partial vacuum in that case. The main culprit for this/that would be rising pistons sucking air through a worn crankshaft seal.
Plus probably pushing engine oil the other way and overloading the primary case with vapour. Which in turn will keep the Triplex chain or belt nicely lubricated but also get in between the clutch plates.
Of possible interest.........my RGM belt drive conversion has now carried me over 22,000 miles. Due to the open primary cover, it get water cooled in the rain, oil from the now worn crankshaft oil seal and dirt, dust and anything else the front tyre can throw at it.
 

When the gearbox it hot it will be pressurised unless it has a breather. Later Commandos have a small brass breather next to the cable entry - check that it is clear, earlier Commandos don't have this. I'm working on a 750 and I've made a filler cover with a small hole drilled in it. Might work?? Just looked, I thought this was my idea! Andover sell them 04.1104
The other trick is to cut the pushrod in half, find a ball bearing that just fits in the mainshaft bore, reduce the overall length of the pushrod by the diameter of the ball and refit it with the ball in-between the pushrod halves and the hardened ends where they were originally.