Howard, you are exactly right! When an oil manufacture gives a specification of 40 as a viscosity, this is measured at working temperature,and how many of our bikes get the oil to that happy situation, but is just what a rattly old engine requires at times-BUT, at start up this old rattler has to rely on oil being fed down a long, cold tube from a very effectively air cooled tank, squeeze it through a felt or paper filter, force it through small feed holes into big ends, cam spindles and rocker feeds, and finally rely on enough of it running back through the crank case scavenge hole to keep the return pump happy. Couple this with the fact that heaviest engine wear takes place during cold start up due to surface drag, and a 10 weight oil would be a much better option, and this makes a 10/40 an ideal choice. You can also buy it anywhere.
There used to be an advertising slogan for paint that read "say dulux to your decorator." I think a similar word should be said to an oil rep that says an oil type is "too slippery", or in other words "too lubricatey". I think the point he may have been trying to make was that a new engine will naturally be used under light throttle settings, and with a super oil installed, this can lead to bore glaze, but I have never experienced this on any of our Vincent engines.
On a slightly different subject, but allied to bore glazing, I have read with interest a recent thread regarding the "lapping in" of poorly finished piston rings. It has been suggested that working them up and down the bore with a lapping compound might be a good idea. It isn't. Any engine machinist worth his salt will ensure that his honing marks left in the bore will be at a specified angle, to ensure that "shelves" are formed for oil to sit on to assist ring sealing and lubrication during start up. This angle is usually set somewhere around the 40 degree mark. Too flat and excess oil is retained. Too steep and the oil runs off or is scraped off and will lead to poor sealing and excess bore wear. Lapping a poorly fitting ring into this bore effectivly undoes a lot of your machinists work,and if any of our mechanics should resort to this action, he would soon be after another job!!
John