Dropping Oil Pressure - Yes I know.....

[AJTich]

Hi,
I fitted an engine oil pressure gauge to my 2003 GU 3.0 litre Patrol.
I use Penrite 5W50 oil, changed every 5000km.
Recently on longer (warmer) trips the oil pressure drops as follows:
a. 2500RPM: 80 PSI drops over time (an hour) down to 60PSI
b. 2000RPM: 65PSI drops over time down to 40PSI
c. after an hour highway driving on taking foot off accelerator, drops to idle and pressure drops down to 10PSI maybe less, sets off my low pressure alarm.

My mechanic reckons it is an oil viscosity issue, so we increased it to 10W50, made no difference last trip out.
He gave it a Wynn's engine flush and changed oil to 15W50 per Nissan Manual.
He bolted in a pressure gauge and said at 20 on mine is 27, so likely all ok....
BUT....why does it drop about 15PSI while driving over time...??
Is it just oil viscosity, or..has someone else seen similar??
Any help, ideas ........ Thanks.