If this is at idle, probably a non-issue... The liquid filled gauges are so much more sensitive to pressure changes you may just have never noticed it before with slower acting gauges.... What is it doing at 1500 and up RPM???? At idle, the oil pressure will bounce around a bit...I suppose something to do with the oil passages in the crank as they rotate and change their position to the grooves and or holes in the main bearings.......With grooved uppers and smooth in the caps, the fluctuation is even more noticeable..... Know maybe somebody can tell both of us why it does this!!!!!!