I think synthetic oil probably does everything oil is supposed to do a little bit better than dino oil, but now that good dino oil is available for my older cars, I'm planning to switch back.

When I bought my '66 Satellite back in 2020, it had a freshly rebuilt 360LA. At the time, there was not a lot of oil available with extra ZDDP (just Lucas and Amzoil, as far as I knew), so I started using Amzoil Z-Rod. Everything works great and I have experienced no real issues, but I seem to have minor oil leaks. The engine is leaking in all of the places where these engines tend to leak, so it may be normal, but this engine only has a couple of thousand miles on it. I also suspect that oil pressure is lower when the engine is hot, but I had not driven the car very much before I did that first oil change, so I can't be sure.

Anyway, my working theory is that the the various gaskets and seals in the old 360 seal better with dino oil than they do with synthetic. I'll put that to the test when get the Satellite back on the road in a few weeks. I also plan to use Driven dino oil in my Vette when I get that running. All that said, I will continue using Amsoil in all of my newer cars. Those engines are sealed with o-rings and RTV, so leaking is not an issue.

 

Synthetic oil does. Not cause leaks.

 

Gaskets do shrink, etc., especially when the car is not run for extended periods.
I think that the high mileage oils have seal softeners.
I've had great luck with this:
I know this is sacrilege to some here, but it sure beats rebuilding a hydraulic system. I couldn't find anything negative about it.
A friend once bought a 25+ year old gasket set. The rubber seals were pretty hard and leaked a lot.

 

People used to add brake fluid to their oil to swell the seals. Why not just replace the gaskets? An oil pan gasket and rear main seal on a C3 Corvette isn't that bad a job.

 

Yeah, you're probably right. I won't be surprised if I change my oil and see no results. After all, my Corvette has dino oil and it leaks worse than my Satellite (it also has gaskets from 1966, so there's that). Still, I'm stubborn, so I won't be happy until I see for myself.

 

The thing that bugs me is that (according to the former owner) this engine was rebuilt in 2018 and had less than 1000 miles on it when I bought it, yet the oil pan gasket seems to "sweat" oil. I have run a borescope into the spark plug holes and the engine has .030-over pistons and fresh looking crosshatch, so I'm pretty sure that it's been to a machine shop recently.