I have a Granatelli monitor, and it shows actual numbers which correspond to a mechanical gauge I installed at one point. While it shows a trend, the in-dash gauge is not accurate by any stretch of the imagination.
I have spent many hours with factory scan tools on all CJD products and while the oil pressure is not real, I have found the temperature gauge to be very accurate compared to the temp sending unit being read by the PCM. Sometimes the cluster will "soften" the movement of the needle, but the temps reflected tend to very close to the actual. Generally any error will be the same throughout the range of the gauge.