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I admire you greatly, Scotch, for your engineering skills and for your ability to impart your knowledge to others.
Regarding the engine speed related oil consumption, could crankcase breathing be an issue?

As with many 1300 owners, I also have high oil consumption - around 1 litre per 1000 miles. I have removed the cylinder head twice, once to check bore clearance/liner wear and the second to replace all the valve stem seals. One area I did notice was that the inlet valves do have an oil film on the inlet port side (see picture). One theory put forward is that the cylinder liners are out of round which provides a poor fit with the piston rings. One would suspect that this would also result in poor compression pressures as well but very few of us see a problem with compression pressures. I recall seeing somewhere on this site a connection between oil consumption and the engine breather system. I work for a company that designs and develops engines and one of the least understood areas to design and optimise is engine breather systems. It seems to be a bit of a black art. There will always be some blowby from the combustion gases into the crankcase which will blow the oil mist in the crankcase out into the breather. A good breather system can trap the oil and return this to the crankcase whilst letting the blowby products escape into the engine inlet system. The design of the 1300 breather system is at the least crude and at best, an afterthought. Oil mist escaping from the breather into the inlet system would at least fit with the condition of my inlet valves. Maybe this is an are to explore further?

Piston blowby can cause a lot of crankcase gases and depending on how you drive, those blowby gases will be hot and cause the oil film on the cylinder/piston to vaporize and or oxidize leaving that brown residue on the pistons and if it's bad enough, black carbon deposits on the pistons stemming from the rings area and developing downwards. Those hot gasses when blowing by, also remove the oil film in that area leaving the cylinder wall unlubricated which further allows premature wear on the piston rings and cylinder walls. For some reason, the pistons themselves don't seem to wear as much possibly due to the porous nature of aluminum which will hold a bit of oil residue.

Looking at your pics Neville, if the inlet valves came out looking wet and oily as I see them in the pic, that's a dead ringer for valve seals. Under normal operating conditions, the fuel droplets in the air mixture which have additives in them, wash the back of the inlet valves of that oil residue keeping them reasonably clean. Valves that come out of a cylinder head and are soaked in wet oil happens when the engine was sitting just before you chose to tear it apart. The oil sitting on the top of the valve spring retainers will creep down the valve stem and [ast the faulty valve stem seals, settling out on the back of the valves. Evidence of faulty seals usually shows its uglyness right at startup with clouds of blue smoke that clear up in a minute or two of running after startup.
Chrysler, back in the early 1980's would not do any warranty work on an engine until the oil consumption went above 1 liter/ 500 miles. When the criteria was met, the engine would be torn down and precision honed to true the bores and new pistons and rings fitted. Kawasaki back in the 1979 and early 1980 models had a problem with oval bores and there was a technical bulletin put out to do exactly the same. True the bores and refit new pistons and rings. And of course, not everyone would have the work done. So it's a crap shoot unless your the original owner as to whether or not the bike you have had this work done to it.

Some interesting comments ! I appreciate the insight.
When I made my intake I decided to NOT connect the crankcase vent to it. My Crankcase breather is a simple 2inch "air filter look-a-like". It's completely dry on the outside as is the area under it - on top of the case. As such I have no concerns about ring blow-by. Regarding "speed" vs. oil consumption: I rarely shift at anything over 5000RPM and generally I will shift at around 4000 RPM. My Hwy. speed for the most part averages 120 KPH to 140 KPH. At 4000RPM I'm indicating 120KPH/5th. As far as my 13 is concerned I can't suggest that speed has any significant effect on my oil consumption. Fast or slow, the oil level seems to drop consistently, regardless. With the issues of oil consumption known to be related to the original factory machining, I'm long past that point of needing to know if my cylinders are out of round. It seemed to me that the most obvious and simplest place to start in an attempt to reduce the oil loss, was the valve seals. Given the symptoms and the recent discovery that the originals were worn-out by 1.5mm and essentially not in contact with the stems, I'm very optimistic I have this issue under (better) control. I'll be happy if the oil loss returns to 1 litre/1000kilometers (and hopefully more mileage.)
I'm mystified as to how this much oil can be lost without a huge cloud of blue smoke, without fouling plugs or without a pool of it under the bike! I'm convinced the seals were the culprits and the oil was merely getting sucked into and out the exhaust. Some oil will obviously be burnt in the header but evidence suggests the oil is not getting into the cylinders. Where's the SMOKE ?
With some recent comments from those who have done compression checks and most have found those numbers acceptable to good to vey good in spite of oil loss - I've adopted a different philosophy. For my 13 I personally feel there is no need to fret over a cylinder or two that might be on the low end of the spectrum. My spark-plugs are mysteriously "perfect". The overall performance I feel is quite outstanding, the start/idle and consistent "driveability" has tempered me to not bothering with a comp.-check. If there were more serious problems/symptoms then certainly a comp.-check would warranted. I'm not the least bit interested in having to invest up to 2G'$ to put it all back together CORRECTLY simply because of "Academics".

If you have bad breathe - You start by brushing your teeth - not by pulling them all out !

Another interesting thread this one !
I've always had similar oil consumption issues with my Z13 - approx .250 ml vanishes every 250-300 miles: like Scotch, no smoke, no leaks, no misfire or fouled plugs; . I could not fathom the root cause, but did a compression test and was getting > 160psi across all 6 cyls. I am a bit reassured that its not an uncommon trait.

I was reading a road test circa 1974, between a Triumph Trident and a Kawa H2 750 (re published in a recent UK Classic Bike magazine) - in that road test the 4 stroke consumed more oil than the 750 2 stroke.

when I using 10/40 oil I was using a pint of oil every 300 mls, switched to 20/50 classic oil ( similar to the 1980’s Duchams 20/50 )
My oil consumption dramatically reduced