oil pressure inquiry

[jusuravragecrakr]

i just recently swapped a fresher ka24de into my 91 coupe.....the motor is an automatic motor......i had an aftermarket oil pressure gauge on my old motor so i hooked that up to the new one......at operating temp..the old motor would idle at 35-40 lb at 900 rpm......the new one idles at 800rpm and has a pressure of 20lbs.....why such a dramatic drop in oil pressure over 100rpm.......it seems to have less oil pressure than the old one under load as well....someone told me that auto motors have less oil pressure....i really don't understand that...but whatever...and someone else told me cuz I'm running 5w-30 synthetic that that could be why too....kinda makes sense i guess due to different type of oil......or an i just tripping out cuz i don't 100% trust this motor yet....please someone shed some light on this inquiry.....thanks :trogdor:

[jr_ss]

Does oil pressure increase as rpms climb? 20lbs at idle is fine. Hell most SRs see 10-15psi at idle. I wouldn't sweat it. Just a note too, oil viscosity will play a small role in base pressure, but it isn't going to be that dramatic.

[fliprayzin240sx]

Actually, viscosity makes a noticeable change in oil pressure. When your oil is cold and thicker, your pressure will be higher. Once the oil warms up and gets to temp, you have lower pressure.

[jr_ss]

I meant as in 15w-40 vs 10w-30, etc. I know when it's could it will be higher than warm but I was referring to the difference between viscosity ratings...