Kudos to me for approximating the pressure loss at 7500' at roughly 3 PSI, off the top of my head without reference to a chart!
After converting units, the chart says it's about 3.2 PSI.
PS: none of this explains why the oil should be thinner at high altitude, due to altitude alone not the related temperature drop.
It's not thinner, not remarkably so anyway, and could be more viscous when up to temp. there's a 20% or so loss in cooling capacity at those elevations.