[Sirrotica]

I agree with a leakdown test. If you've done nothing to lower the stock compression in the engine, it is likely detonating on todays fuels. Even 94-93 isn't enough octane to stop detonation if the OEM heads are still on the car. To run that engine properly, you need fuel in the 95-96 octane rating range.

Once detonation happens it hammers the top compression ring, being quite brittle they crack under detonation, then you have the ensuing blowby that you describe. In the 70s when they did away with leaded premium fuel, this was a common occurance in the 60s high compression engines. They finally brought some premium unleaded fuels back, but they don't support the advertised 10.5, or 10.75 to 1 compression ratio your car has stock.

I had a 1965 389 from a Bonneville that suffered first cracked rings, and finally a broken piston due to detonation, with insufficient octane fuel. Even if the rings havent cracked, the detonation hammering away at them takes the tension out of the compression ring causing it not to seal the cylinders as well. There is course wear to also be considered, but it is difficult to get these old engines to run correctly on todays pump gas, without lowering the compression ratio.

Using octane booster is pretty much a waste of time, because the little bit most of them raise the octane, isn't enough to stop detonation. Usually the only way to get enough octane is to blend some race fuel with every tank of premium unleaded.

Running a leakdown test will tell the story of which cylinders are low, and how much damage, or wear, has occurred.

The car must have sat awhile without being driven, then once it was driven the lower octane fuels commenced damage on the engine. The rings that GM used during that era are quite brittle, and it doesn't take a lot of detonation to destroy the sealing.

There is an oil additive that can help slightly if the rings are just worn, and not cracked, it's called Restore. I have used it in worn engines and it will help slow blowby if it's just normal wear. It won't help if rings are broken though. It's only a bandaid on worn rings, and cylinders, at some point it still needs to be addressed mechanically.