Unlikely to be a head gasket problem, as any weakness which allowed combustion pressures into an oilway would also result in oil passing back into the cyls during the non-power cycles => you would be burning/losing oil noticeably.
In my mind it is either:
(a) normal (but it does sound too high), or
(b) a gauge calibration problem (ie reading inaccurately), or
(c) a pressure-relief valve malfunction. Oil pumps need to generate good pressure at very low rpm and so usually produce excess pressure at med/high rpms. I'm not sure what pressure blow-off system the 456 has, or how accessible it is but maybe it's a bit sticky.