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Zinc content in oil

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K2-K6:
I feel like that as well, have to think about one part of it at a time else brain goes to overload  :)

It's partly right in that if you keep the oil in good condition you are less likely to break through its structure. But that is balanced by some engines having a camshaft loading so high that the oil, even in new condition,  will only just about survive the loading it takes.

Fortunately the SOHC fours have well designed and considered valve gear geometry on the whole which will mainly not give problems, but, it's easy to make any engine step over that limit. One way is to have valve clearance too small,  this does not allow the oil film to re-establish on each rotation of the cam so the depleted oil supply as the follower comes up on the ramp it breaks through the oil and gets metal to metal contact.
Another easy way is to keep running the bike alot on choke with no long journeys. The oil is quickly diluted with part burnt petrol and other contaminants from rich combustion, this very quickly trashes the oils shear properties and offers low protection to the cam.

Bryanj:
Don't forget when we use the oil it is getting hammered as gear oil as well

K2-K6:
At first appearances it looks like that just because the gearbox looks so gnarly, it gives the impression it would badly affect the oil. But different gearboxes are designed to run on different types of oil which if done correctly will not adversely cause any real harm to the oil.
Most gearboxes with their own oil will run many thousands of miles without changing the oil,  it's the combustion by-products in mixed systems that prematurely degrade the oil with the risk opposite to how it's commonly perceived ie, dirty diluted oil puts the gearbox components at risk.

I've thought through about your point Mick,  about anti wear properties (talk about taking one thing at a time) the oil film is likely to be breached each time a motor is started. When it's turned off and left then the oil that was in the bearings partly leaks away,  then when you go to start it again you are reliant on the anti wear properties to prevent damage and metal pickup until the oil pump has pressurised all the bearings again. As soon as that crank starts rotating and the big end gets thumped by the compression it won't have the desired pressurised oil film to protect it. The cam is also under valve spring pressure / load right from the off, whether any oil is there or not.

totty:
Isn't separate gearbox oil which is ran for high mileages normally single grade?
I'm sure I'd read somewhere that gearboxes break down some types of viscosity improves used in multygrade oils, causing the oil to get too thin at high temperatures.

There's another consideration but I'm not sure if it's just marketing hype, which is extreme pressure (EP) additives. Gearbox oil is typically marketed as EP oil, they used to market specialist EP oils for A Series engines due to them being one of the few cars with the gearbox sharing the engine oil, and it used to be one of the justifications for bike specific oils being required.

K2-K6:
Each gearbox would by designed from scratch to use a different type of oil, an ep oiled box and most differentials will take advantage of that level of protection by asking it to transmit a higher level of torque for a given component size and ratio change.
You're right that gearbox specific oils are often monograde or restricted viscosity range eg 75/90. But one of the main differences is that gearboxes mostly don't have a pumped system of distribution so that you don't run into cold oil hydraulic locking the pump as all the components cope with torque ratings far in excess of that needed to move sticky oil around. It'll just cause drag and power loss until warm.
Compare this to an engine,  they have to pump oil round to all components so by default need to run a pump. Most engine oil pumps if you started with mono / hot grade oil 40 for example will be at risk of hydraulic pressure overwhelming the oil pump drive if you started it in zero degrees, this is why before multigrade was available you'd have to tailor your oil viscosity to cope with seasonal temp variations.
The Mini combined system may have benefitted from a heavy duty or extreme pressure denoted oil as you say but I think this is nothing more than a zinc heavy bias to the additive package, like the original post title asks about.
There are other examples of boxes running engine oil. The original range rover 4speed was designed from scratch "lt95" to do this and incorporates a pump to lubricate it, you can't run something like ep90 in these for risk of shearing the oil pump drive.

The add package components in transmissions oils I don't think are in any way suitable for use in engines. They use things like sulphur, that although being resistant to some heat in normal use won't tolerate direct surface contact in places like cylinder heads as I understand it.

Something else component wise that look like it benefits from ZDDP are the internal chains, both cam and primary drive. The size of the bearing surfaces and torque going through the primary are candidates for bringing oil film on its own to the very limits of protection via  the pressure they're able to exercise.

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