I just purchased an 89 IROC with the 350 TPI. Vehicle is all stock with 77K. Previous owner was using Valvoline conventional 10W30. Vehicle had great oil pressure (30-60). I took it in for a full tune up an flushed out all oils replacing with synthetics. I used Royal Purple 5W30 with a NAPA Gold (Wix) oil filter. I took the car out for a cruise yesterday and the oil pressure kept dropping till it was finally at 12 to 14 pounds @ 2000RPM and ZERO at idle. I turned it off and let it sit for an hour. When I started back up, it was a little under 30 but kept dropping till it was again at zero when idling. Oil level was perfect on the dipstick. I'm not experiencing any pings, knocking or any other abnormal noises and the car runs extremely smooth and strong.
What seems to be the issue? Oil weight? Sending unit? Pump? Will Seafoam help? Any ideas?
I used the same oil and filter in my 87 Vette with the similar L98 and 80K on the odo with no problems. Any help would be appreciated. Thanks.

 

Are you 100% sure the previous owner used 10w30??? And not a higher weight oil...

Plus, flushing out the system sometimes makes sludge in the oil pan clogging the oil screen...

 

Please post a specific problem only once, and in the correct forum. You already had this posted in the 3rd Gen tech area, where it belongs. "Drivetrain" is for topics like clutches, trannies, rear axles, etc.