I use a multi-weight synthetic. And, as long as I register pressure, along with return to the tank, I see no real advantage of one over the other. Which is to say, I'm really giving an answer to your second question. I start out with the factory setting on the pressure regulator. And, only tweak it up when the engine is hot, so that the oil pressure light stays off at a moderately fast idle. Then, I don't touch it any more. Never seen the advantage of a stronger spring. If the engine is worn to the point that you can't get what you need out of the stock spring, it's time for a rebuild.