A warning about stp oil filters
I bought an oil change deal at Autozone for Quaker State 10w30 oil and an STP oil filter for my 65 6 cylinder 200/auto. I had gone about 4000 miles on the old oil and needed to go to Houston which is about 80 miles away. After changing the oil /filter, I cranked it up and ran it for a few minutes and turned it off. I looked at the oil pressure when it was cold and I noticed it looked about 5 psi lower than normal. What was also weird was that it took awhile to build up to that pressure (about 45psi) whereas before it would climb right up and hit between 50 and 55 depending on outside temp. I didn't have time to do anything about it, but I kept it in the back of my mind as I was driving.
When it is warm the oil pressure will stick around 35 at 2500-2800 rpm or right at 70mph. On this trip it kept dropping down to 25 psi. my coolant temp hit 190 which is normal on a warm day (80+). When I would come to a stop the oil pressure dropped down to 5 psi with it in drive. I put it in neutral and it went up to 15, which on my car is low. I normally have about 20psi at idle with the car in drive at temp. First chance I had I pulled over to check fluids. All good, oil clean and at full, coolant was at the right level as was the tranny fluid. I limped home at 60 pressure at 25.
I noticed the stp filter was shorter than an equivalent motorcraft or fram so I decided to change the filter and leave the oil. Using an old racer's trick I used to buy the 2 quart filter for my 75 chevy pickup and it was good for 10 more psi over the stock filter so I figured using the longer motorcraft couldn't hurt. Sure enough just swapping the filter brought the oil pressure back up to 55 psi cold and gave me a running pressure of 35psi at 70. the pressure at a stop in drive at temp was 20, like it had always been. More importantly the pressure climbed back up like it used to instead of laboring up.
I knew I was going cheap but I didn't think it would restrict the flow like it did. I don't think my motor was damaged as I have driven a couple hundred miles since then. I got lucky. Long story short, If you are running an STP filter I would change it immediately.