I am familiar with Zee0Six, I have read his PD pump & filter writings. We have discussed when PD isn't PD anymore.
Let's say the main oil galley coming out of the pump is 1/4" diameter. There are smaller passages coming off of that, say some are 1/8", some are 1/16th. What I'm saying is the PD pump (in your extreme example, high pressure & high volume with extra thick oil (20w)) will fill the 1/4" galley quickly, but the smaller orifices are not getting oil yet, because until the oil fills that galley, its pressure is effectively zero. Full flow with zero resistance = zero pressure. So then it fills the 1/4" galley, & sees pressure, practically instantly, now the oil is attempting to get through the small orifices. Lets say the relief is set for 85psi, but the thick oil requires 90psi to get forced through the 1/16" galley. That galley will starve until it warms up enough to flow through with 85psi. If the galleys are full then the oil attempts to pass through the smaller passages instantly, pushing the relief open.
Extreme example but it illustrates my point that a thinner oil will circulate faster than a thicker oil. In real life with 10w-30 at 104F the difference is (very) small but still a difference nonetheless. The PD pump will force the oil through the small passages but the back pressure may be slightly higher with the 10w as opposed to the 0w. I agree if the relief is closed, the pump is pumping the same volume no matter what. But the 1/16" passages may be getting slightly less flow because the 1/8" passages are taking more of the flow. A thinner oil requiring less pressure to flow through those 1/16" passages will deliver more flow.