High oil pressure means one of two things:
1. The oil is cold
2.The oil pressure relief vale is either sticking shut or has the wrong spring.
When the oil is cold it is too viscous to lubricate properly (it won't wet to the surfaces and form protective films). That's why they tell you to wait for the oil temp to reach the operating range before running at any significant power levels, and it's also why high pressure is acceptable until the oil warms up (but noting that you shouldn't be running at much more than fast tickover until it does). oil pressure/temperature errors in very cold OAT are a concern because they may indicate that the thermostat to the oil cooler is stuck open, so the oil is not warming up and may even get colder (more viscous) when you climb into colder air. That way lies a high risk of increased engine wear/damage, and a moderate risk of catastrophic engine failure.
If it's the oil pressure relief valve that's stuck shut (or has too strong a spring due to an manufacturing/overhaul/maintenance error) then you will subject the oil system to too higher a pressure, and risk damaging the oil filters and/or blasting out any core plugs in the oil galleries. In either case the risk varies from a persistent oil leak and lack of filtering to a sudden and catastrophic total loss of oil when a core plug fails and all the oil makes a break for freedom through the resulting hole.
If this is a new or newly overhauled engine which has been like this since installation I'd be taking it up as a warranty issue with whoever supplied/overhauled it, and I wouldn't be happy flying it. I'm actually surprised that the RTS has been signed with that kind of defect.
Even if it's just an instrumentation issue it's a no-go item IMHO because, as you say, this means that the pilot cannot rely on one of his/her primary engine health indicators.
£0.00007 supplied,
PDR