When fuel goes bad, very bad things happen.
I started and ran Isabella with bad, oxidized and varnish-y fuel in the tank, which caused many problems. It gummed up the EFI system components requiring the metering valves, control pump, fuel lines, and fuel tank to be cleaned. The fuel, as it passed though the intake manifold and cylinder head, left a trail of varnish. Worse, that varnish coated the valve stems and seats.
The next time I tried to start Isabella's engine, the varnish coating the engine's valve stems and seats had hardened cementing the valves in the positions when the engine was last shutdown. The varnish prevented the valves from opening or closing. The starter still tried to turn the engine over, and when the valves wouldn't open, the starter's torque bent pushrods and broke rocker arms. This necessitated a valve job.
I removed and sent the cylinder heads out to be reconditioned. That solved the problem of the stuck valves. I also sent the intake manifold to be tanked and cleaned. I replaced the rest of the valve train including all new rocker arms, push rods, and hydraulic lifters.
Despite the age of the engine, I was happy to find, despite only using conventional motor oil, the engine was pretty clean inside. I've switched her to Mobil 1 10W-40, a good choice as that oil has boosted ZDP additives that will be better for my flat tappet lifters and camshaft.