What's happening is the normal rock-steady 50PSI oil pressure steadily declines over the hour long climb down to 30PSI. Speed never changes, water temperature never rises, just the oil pressure declines. Once the summit is reached the oil pressure recovers over several minutes back to exactly 50PSI.
Having discussed this with five oil company reps several say 'this isn't viscosity breakdown because the pressure recovers'. I tend to agree. I think this is more along the lines of the oil pressure regulation actually causing the pressure drop because of either adverse thermal response or from a small (normal) viscosity drop.
Oil reps are suggesting the need to blend synthetic oil with non-synthetic at this point.
kcress - http://www.flaminsystems.com