Oil Pressure problem or oil sending unit

I have an 05 Pacific 3.5L with 150k km. The oil light has has started to come on upon start up in the morning, but if I turn off engine and start again immediately the light stays off all day. I checked the oil level and it is fine. It is time for an oil change. Normally I go to Mr. Lube and get the house special but maybe I should think of a higher quality oil and higher quality filter? Maybe the house oil (10W30) loses viscosity over time and miles and the cold start up doesn't have the required pressure so the light gets tripped, then when I do the second start the pressure is there and the light goes out.

Any thoughts on oil quality or filter quality causing this problem? or do you think the sending unit is causing the problem? I'd change the sending unit but it is hidden behind the idler pulley on this model so there is a fair bit of dismantle required to change it, so I don't want to do it just on speculation.

Good news. Problem solved. It was the oil sending unit/sensor. It was a bit of a pain to change but the light stays off now.