Wouldn’t the heat rise along with the vapor and Heat the Cam etc. Particularly in
a cowled engine with a blanket cover?
Could a difference in the materials between brands play a role?
The heat rises all right, but it is wicked out via the cold case walls. So the top is colder, because only the sump is heated. A car warmer inside the cowling heats the entire engine from the outside, meaning that it gets plenty warm up top, discouraging condensation on anything inside the top of the case.
Cams and cranks are made of high-carbon alloy steels. That stuff corrodes more easily than low-carbon stuff. Not sure of the electrochemistry there, but I've seen it.
Then they're mounted in dissimilar metals. The crank is in lead/tin/copper-lined steel bearing shells. The cam runs directly on the crankcase aluminum/magnesium. Get a bit of moisture in there and you have electron flow that breaks down the oil and combines it with H and O from the water, and you get acids.
Lycoming and Continental have been building this stuff for a long time. If there were alloys that were much more corrosion resistant, yet at least as strong and resilient, they'd be using them.
Everybody worries about the cam, but I have seen serious cylinder wall corrosion and rusted gears in the accessory case. These are also fatal to engine life. The piston is a high-silicon aluminum, running in a 4130 chromoly steel cylinder. Moisture gets in there too, and causes cylinder pitting, which accelerates the cylinder wear as the pistons run across those pits, and the engine starts using oil and making metal. Those pits harbor a lot of oil that then gets burned when the cylinder travels downward on the combustion stroke.
Moisture in the sump corrodes it. The thin stamped-aluminum sumps on the big Continentals are known for corroding right through.