Talking oil pressure, I used to own a '57 chevy station wagon I used to drag race. I had built the engine up to around 400 hp. The night before race day I would remove one of the valve covers then start the engine at idle. I had stretched the oil pump spring and added a couple washers behind it, that gave me 40 psi oil pressure at 1250 rpm idle, and 60 psi when I revved the engine to 8000 rpm in each gear when racing at the track.

During that cold engine start up at the house I would wait and wait for the oil to pump up though the pushrods to lube the rockers and tops of the valves. I remember it would take up to 4 minutes for this to happen. Back then the oils used were 20, 30 and 40 weight, no multiple viscosity oils till a few years later. Once the oil appeared and the engine was hot enough, I would shut off the engine then set the valves on both sites using a setting chart from a Chevy shop manual. I was using 30 weight red top Valvoline back then.

So it's easy to see why toyota wants the 0W-20 in their hybrid engines. It's a high viscosity oil only that adds a few mpg, but also gets up into the engine at a fast rate.