when I start my car in the morning my oil pressure is good 60-80pds - i get on the highway I usually ride between 3800-5000 rpm (pending on how late i am to work) well anyway i can be driving when i first get on the highway at 4000rpm pressure is at about 75-80pd but as the car keeps going pressure at the same rpm slowly goes down to 50-60pds...on my friends car his always between 60-75 atleast but not mine...I dont think its an oil pump problem cause i had this exact problem b4 using an ls pump and b16 pump(the b16 was off a 2000 si) I also tried a motor flush and that seemed to help and kept pressure good for a while but that has dosent work anymore...seems like when the car is cold everything is ok but as it gets hotter pressure drops...help me please b4 something bad happens to my motor

 

You want to have at least 60psi of oil pressure at 5000 rpm. At idle you should have at least 10, but 15 is more common. Oil temperature, oil weight, and rod bearing clearance could all attribute to the difference between the two engines that you mentioned.