That's all most interesting Dave and makes perfect sense to me. I.E with thicker fluid the pump seals better and the injectors etc get the pressures they need to perform as they should.
I've had the injectors reconditioned with new Denso parts. The test before showed poor injectors with one leaking badly. I was meticulous in putting them back not to get anything dirty though I didn't flush the pipes, pump or anything else for that matter. I can only assume that the injectors are still fine and within spec.
Compression comes up as 500psi +- 20psi
Nothing is loose or degraded on engine mountings or dampers.
The only thing I can think of is that the pump has a slight problem on one cylinder, perhaps a leaky delivery valve.
Symptoms are worse when the engine is cold and just up off idle. Cold weather makes it worse and has on the coldest morning shown some white smoke from the exhaust where normally there is none.
The video below is more for the audio really. Just after start, you can, if you listen carefully, hear what sounds like a regular knock which in my mind is one cylinder out of the 6. BEBs have been changed and this fault was present before and after, unchanged.
https://vimeo.com/162047394
By process of elimination, and by the fact that nothing I've done has altered the problem for better or worse, and that allowing better airflow (K&N filter) seemed to make it slightly worse, my thoughts are with a lower delivery of fuel into one cylinder. I suspect a delivery valve on the pump to be leaking allowing the standing pressure in the injector pipe to drop after each injection, something the subsequent injection has to make up before the pressure appears at the injector, causing less than the intended measure of fuel to be delivered.
My next planned move, when I get the chance, is to remove the pump for test and potential rebuild. I don't like condemning anything before finding it is at fault so a test should hopefully show up whether or not the pump has a problem. Getting a test done on a mechanical pump these days is getting more and more difficult (or so in told).
Hopefully better weather is coming so it may be time to bite the bullet soon and heave the pump out for test.