Hello guys need some help here. I changed my oil today to royal purple 20/50 with a napa gold filter. I'm not sure what oil it had before this change. The pressure with the old oil would build up to 80 psi at start up and drop to about 25 psi. It would build up as the rpms got higher. The new oil got to 80psi and dropped to 40psi as the car got warmer. I decided to take the car for a spin and got the rpms up to about 6500. It was fun until i noticed the oil pressure dropping to 20 at the light. I put it in nuetral raised the rpms to about 2000 and the oil pressure would build up again but drop just as fast. When i got home the pressure dropped completely to about 5-8 psi. What could of happened? What type of tool can i use to test the oil pressure directly from the block? Appreciate all comments!