What wieght oil do you use?

For some unknown reason even my machinest couldn't figure it out, I only had 5-10 psi oil pressure using 10w-30 valvoline ( courtesy of the dealership I work for) Change the gage and then it showed 10 psi fully hot. I used regular Mobil 10W-30 and it went to 15 psi. Then I tried mobil synthetic 10W-30 and then my intake start leaking oil. I just put in Extended Performance 15W-40 by Mobil, now I have 25 psi full hot. I've got about 8,000 miles on this motor. And about 250 1/4 mile passes and plenty of high rev highway driving. No problems. It's a 97 360 Magnum in my 63 Valiant. All I know is the synthetic does not break down by excess fuel like conventional oil does. When I was racing every weekend and driving it during the week I used to change it every other weekend. Now I go all summer long and still looks clean! Even this winter when I would start it up to move it around the garage until I fowled the plugs. That's my 2 cents worth