Change in viscosity due to temperature.
The blue line is what I recorded; five samples. It represents the change in viscosity, as reflected in pressure at the main, with the pressure at 100° being 100%.
It follows the theoretical curve pretty well (theoretical data is widely published, I chose the data from Viscopedia for 15W-40) until I get into the lower pressure readings.
The gold line (graphed against the right axis) represents what pressure should be seen at the main, assuming that the pressure in the main at 100° is 25psi (as it is in the 'Crank Control' data above, which represents the sum of all mods).
I consistently see higher pressures than the theoretical data would predict, especially at high temperature/low viscosity. I can think of two and a half reasons for that.
1) My gauge for the main oil pressure is off to the left and there could be some error in reading the pressure in the video due to parallax. I tried to compensate for it, but could have been off.
2) The gauges I am using are not the best quality and may be slightly in error at lower pressures.
1/2) There are some dynamics going on in the KLR system due to the apparent pumping that is done by the crankshaft, as originally suggested by Paul. It becomes apparent when the feed to the crankshaft is restricted, but it must be present under all conditions. It may have some effect on the main that becomes apparent at high temperature/low viscosity.