About 20 years ago, I switched over to Mobil 1 10/30 in my street/bracket car. It had a well worn and loose 440 6 pac that only blew a little smoke out the pipes on the big end. It also had enough blow by to lift the dipstick up a tad at the 6k shift point. Long story short, I changed to synthetic before a trip to the track and on the way, I showered down on it and when we got to the track, my buddy who was following behind me had fresh clean oil spots on his hood and windshield, I also had them on my back bumper. The synthetic was so much more slippery that it was getting by the rings, I went back to dino and it went away. I run synthetic in my modern cars and it is great, but if your old engine is well worn it might not be a good choice! Needless to say, I rebuilt that engine the next winter.