The 360 oil system is really quite simple. An engine driven oil pump circulates lubricant through a series of galleries and oil ways/ports. The oil finds its way back to the sump via gravity and the pump lifts it and pressurises it again.
To prevent too high a pressure, a pressure release valve (prv) is fitted. The prv is sited on the rear of the right hand crankcase above and behind no 3 cyl. It comprises a stainless steel ballbearing and a spring. The ball is forced by the spring against an oil port. When the oil pressure is sufficiently high the ball if forced off its seat and oil is allowed to bypass this restriction thus making sure that the pressure never gets too high.
My first suspicion would be that either the prv has been adjusted to improve pressure or something is preventing the ball from becoming unseated. It's usually a yellow coloured housing with from memory a 7/8" hex as part of the casting. It should also be wire-locked. Not sure how much maintenance you are allowed to do in yr country but your engineer will be able to inspect it in a a few minutes.
Too high an oil pressure is just as bad as too low an oil pressure. 90 is at the top end of the Lycoming scale.
Hope that helps.