I have an Oldbritts oil pressure gauge fitted. What I have found interesting is that on mine, sustained high speed cruising does drop my oil pressure below the 10psi per 1,000 rpm, and when it gets to about 30 psi @4000rpm, I back off for 5 miles or so and up goes the pressure again, as the engine is able to get rid of the excessive heat, and the knock on effects to the oil through not working so hard.
In all probability, had I not known that the pressure had dropped and I had continued, probably no harm would have come to my engine, but it has made me aware that you can get your oil very hot.
I also have lost all of the damping fluid from the gauge (glycerol I think). I must have knocked the rubber bung when it twisted in its housing as the connections were tightened, but it works fine with no oscillating of the needle.
Oil cooler next maybe?