It's normal
I did a speed test run before parking the boat for the season. Normally cruise at 1300 rpm, 33% load, 47 psi. The run up to speed noting figures every 100 rpm took maybe 20 minutes, ending in 3050 rpm and 99% load. I returned to 1300 rpm and the oil pressure dropped to 42 psi. Over the next several minutes it came back up to 47. Oil temps were up slightly during the run but not much (10 deg). I had noticed this the one other time I ran for a short time at full throttle as well. Oil was just changed, Valvoline Cummins Blue 15W-40, as called for by the book.
Foaming? Anybody else's engine do this? Cummins QSB5.9 380hp.
A momentary 10% drop after running at 99% load and then returning to 33% load? All engines will do this. It's normal and expected, and really, beneficial, because it indicates more oilflow at heavy load.
Thermal expansion is the primary reason, reduced viscosity at high temps is also a factor.
Under heavy load, (even though you won't see it in your coolant temperature), the oil-lubricated, load bearing surfaces inside your engine produce more heat, and their temperature rises. This heat does get dissipated through the cooling system
eventually, but remember that heat and temperature are two different things.
As the parts that are under friction loads are heating up and expanding, this expansion creates more clearance for oil to flow through. (remember that as the components of your engine heat up,
everything expands -- including the clearances). Given that your oil-pump wants to deliver a constant volume of oilflow, increasing the gaps that the oil flows into
increases the volume of oil flowing and this is why the pressure drops.
Also, at the point(s) where the pressurized oil is hitting the hot bearings, the temperature rise is dramatic and oil will definitely thin out there, so this too increases flow which results in reduced pressure.
Either way, increased flow under heavy loading is a good thing.
After you back off on the engine load, the situation reverses. Metal parts cool down and contract and oil clearances return to normal and that's why the pressure goes back up.
Re:
"Oil temps were up slightly during the run but not much (10 deg).."
Your lubrication system is also a cooling system. Just because the oil temp at the sender is (let's say) 160 degrees doesn't mean that the oil inside the bearings isn't
much hotter. The oil carries this heat away from the bearings and (in most engines like yours) this is why there is a separate oil cooler. That 10 deg rise you measured at the oil temp-sender may have been 100 degrees inside the bearings.
Plenty more here:
https://www.google.com/search?q="connecting+rod+bearing"+"flow"
Just curious, what was the oil pressure at 3050rpm? At high RPMs I generally see something like 60-70psi on my 6BTAs, a little higher for the 6LYA.
. The displacement mode cruise is a little better than half as fast, and over three times as fuel efficient. 
