Just taking stock so far, it seems the monograde/multigrade debate can be crudely summarised as follows.
Ideally, oils would have constant viscosity (thickness) independent of temperature. So on a graph of viscosity against temperature an ideal oil might appear as a horizontal line as shown below.
However, in practice, viscosity does not stay constant. It decreases (exponentially) as temperature increase and this is true for all oils. So if we draw a graph of viscosity (on a logarithmic scale) against temperature, we get something like the sloping lines shown on the diagram below (not drawn to scale though as this is for illustration only).
For a monograde oil such as SAE40 we get a steepish straight line sloping down from left to right. Note this line intersects the one depicting the ideal oil at approximately the operating temperature for which it is designed (say 100 C). The problem is that at low temperatures, this oil will be very thick so drag will be excessive and lubrication at cold start may be poor.
One answer is to use additives to help flatten the viscosity line (i.e. reduce its slope or increase its viscosity index) so that it lies nearer to the ideal horizontal line. Hence at lower temperatures the oil behaves more like an SAE10 whilst at high temperatures, continuing to behave like an SAE40. This is basically what multigrade oils do. Hence they have titles like SAE 10W40 to designate that they behave more like a 10W grade when cold in winter but revert to SAE40 at normal operating temperature.
The potential advantages of multigrades are thereby evident, especially during starting and at low temperatures. So what, if any, alternative advantage can monogrades potentially offer? I have seen it stated that:
1 They do not deteriorate (thin out excessively over time at higher temperatures) to the same extent as the multis do.
2 Monogrades are less prone to sludging since they do not contain the VI additives.
3 They are less prone to wetsumping (important within our current context).
This makes sense to me and fills in a bit of background. Unfortunately, it still does not definitively tell us which is best! Perhaps the relative advantages cancel each other out so that, as some on this forum have said, it does not really matter which we use, as long as we change the oil regularly.