Probably the gauge sensor. I believe the ISC has two senders, one for gauge & one for ECU.The following from my notes taken off other iRV2 posts:
Oil Pressure Sending Unit: for 1998- VDO 360009 located forward of the turbo, aft of the location for coolant “filter” and on top of the oil filter housing. To diagnose VDO sender, ohms w/engine off should be ~10, with ohms increasing as oil pressure increases up to vicinity of 240. For non-VDO senders, engine-Off ohms ~240, with ohms decreasing as pressure builds.
If that's how its always been since you've owned it, could be somebody replaced a VDO sender w/a non-VDO?
Baja-tested '08 2-slide 36'
Alpine: The Ultimate DIY'er Project