I’d start off rather suspect, factor that in with price, a varying scale based on how much of a pre-offer inspection you do. Even the ‘dry southwest’ has rain, snow, and temperature swings. I’ve seen plenty of heavy condensation when warm air moves in at a much faster rate than cold-soaked metal warms. That is of course the outside of the engine.
One of the byproducts of combustion is water. Lots of water. I read that burning a gallon of gasoline can generate a gallon of water. When an aircraft engine is cold (meaning not up to operating temp), the clearances in the cylinders are large, and there is blowby of combustion gases into the crankcase. Those gases contain corrosive agents as well as large amounts of water vapor which condense in the case, where the water mixes with oils and in the presense of catalyzing metals, it forms acids that eat the engine. This is the reason why manufacturers recommend that the engine be flown for at least an hour rather than just ground-running it; it takes that long for the clearances to tighten up and the oil to get hot enough to drive off the moisture through the breather.
I have run up engines for five or ten minutes before an inspection, then upon checking the filter, find metal. So a cylinder comes off to get a look at the innards, and I find water in the rocker cover, water between the piston and cylinder, water everywhere. This was in engines in a flight school, where they did not sit for days at a time, never mind months or years, and it was a fairly dry climate. There are articles on this stuff on the 'net, too.
Someone on POA a while back calculated the amount of water in a cubic foot of saturated air. It's very little. Microscopic. This was in a discussion as to where water in fuel tanks comes from. The calculations included the amount of water that would enter an empty tank at night, and assuming that the air that left the tank in the morning had NO water in it and had left it all behind in the tank, how long it might take to leave a cupful in the tank. It was forever or something. The point was that the water comes from sources we don't suspect: the fuel truck, rain and snowmelt getting past the cap gaskets, and so on.
An engine crankcase has very little volume to it. It's small enough to start with, and is filled with crankshaft and camshaft and con rods and stuff. The mass of the engine damps the temperature swings in it. Very little crankcase corrosion is attributable to atmospheric moisture. It comes almost entirely from combustion gases.
Why don't cars suffer this problem? Lots of people drive very short distances. Old cars did have this problem, though. They had open crankcase breather tubes like aircraft engines do, and the same acids due to combustion gases, and they'd rust out. In the '60's and on the PCV crankcase ventilation system was installed, to reduce pollution levels, and engines suddenly started lasting a lot longer. Filtered air was constantly being pulled though the crankcase to remove nasty gases, and they also removed the water vapor, leaving the crankcase fairly dry. Remember that this was pulling vast amounts of atmospheric air though the case, with its water content and all, and it still cleaned up the corrosion problem. Besides that, liquid-cooled engines, especially modern engines, have really close tolerances and don't blow by much combustion gas at all. The PCV system won't work on an airplane; it relies on manifold vacuum to work, and with our engines set at 75% power there's not enough vacuum to suck the gases through the system. An absolute vacuum is at 29.92"Hg at sea level, decreasing with altitude, and if you're running at 25 squared or whatever you have almost no pressure differential between the manifold and atmosphere. A car's MPs are often at 12 or 15" or less.
https://www.lycoming.com/content/frequency-flight-and-its-affect-engine