righto... is anyone as confused as me about what this guy is talking about with fuel pressure?! You obviously meant oil pressure.
your oil you are using is probably a bit thick on cold nissan quote a 7.5W/30 i think from memory, so you are well above this. Try somehting like a 5W/40 or possibly a 10W/50 from what ive tried they are both fine and do the job well. Make sure they are fully synthetic aswell.
Whether its the problem to do with the oil...i doubt it but changing your oil wouldnt hurt you in this case. Dont go by teh stock oil pressure guage either, they are prone to irregular/incorrect readings depending on the age of the car/sensor. Chances are you've got a vaccum leak somewhere, or something along those lines and this is causing your idling cold start issue. Try using the Search feature and look for idling issues and im sure you will find something that will help you out.
Have fun
David