General rule on oil pressure that I learned in school, You need 10psi for every 1000rpm of engine speed. Most manufacturers figure in at least 30-35psi extra. For my 350z the service manual states that at idle it should have 15psi minimum and at 2000rpm it should have 40psi or like 42psi I cant remember off the top of my head.

As far as your steering question, Im not sure about all the different models of the 300z but youd have to get a rack that has a smaller ratio for your desired "less lock to lock"

And for your idling question, there a number of things that it could possibly be, check vacuum lines, make sure you have no intake leaks.