OK, IF your rpm is not dropping when you go up the hill the only thing that will cause an oil pressure drop is heat. you climb the hill, the turbo spools up and works a bit harder and the piston undersides get hotter and this heat gets dumped into the oil causing it to thin out a bit. Maybe, you have an oil cooler issue, but try the simple stuff first, read on...
The reason I asked the other questions, the oil when it leaves a hard working turbo (on just about any engine) is almost pure foam. IF you have a low oil level, sometimes the additives in the oil that de-foam the oil when it gets back to the pan are not able to do the job and the oil pump might start to suck in a bit of foam (air) which would make you loose pressure. Likewise, if the oil level is too high, you have the turbo making foam AND the crankshaft lobes will be churning the oil too. This will also cause oil consumption.
Rotella is good oil so changing brands is not the answer. I would recommend making sure that the dipstick is correct for that engine. Maybe...you are over filling it without knowing it? The dealership can look up exactly how much oil is supposed to go in to the engine, and then when it's in, see where it lands on the stick and re-mark the full line.
Also, be sure you are allowing adequate time for the oil to drain back when checking it. The most accurate reading you will get on a dipstick is first thing in the morning after a night of sitting ( on level ground too!) Checking your oil when you fuel after a, say, 5 minute shutdown will not give you an accurate reading.
If this only happens when climbing a hill, that rules out a lot of other things, so start with the simple stuff first.
