Can't go wrong with the PennGrade. It's the most stable on a mechanical gauge that I've found. It is pricey, though.
What motor? I assume a sbc 305, notorious for valve guide issues along with flat cams till the early 80's. I think about the only thing STP oil treatment is good for is to thicken oil to increase pressure. I think pretty much all our cars called for a 10W30 or 10W40 when new, 5W20 is a thin choice. Maybe the thin synthetic removed sludge? Usually valve seals issues on older motors, replacement is the only option. They are hard plastic that disintegrates. I use Kleenflo Combustion Chamber cleaner and it works great to free stuck rings, used on a couple of motors. Gained compression, 15 to 30 psi and less oil consumption don't lie. I got caught filling cylinders last time, bent one rod and destroyed the rod bearing. I still gained back 15 psi on that cylinder. Interesting that it worked.I have owned my regal for 10 years. Since I have had the car I used Lucas Classic car hot rod oil. For 10 years the valve seals where dry so when I use to start my engine it would blow blue smoke. Just for a second or two. It was getting pricey. This time I always use to use Pennzoil so this time I went full synthetic 5W20 and a bottle of STP oil treatment. Not only did it lube the valve seals, no more blue smoke on start ups but I have 1200 miles and have not added any oil. Turns out my oil consumption problem was due to the valve seals problem. I keep looking for leaks but none so far. Hope this helps.
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