Most of the moisture is not getting in from the outside, it is water from the combustion process. Burning hydrocarbons mainly produces water and CO2 and because of the old design of these engines there is quite a lot of piston "blow by" and the combustion products end up in the crank case. The water then reacts with the some of the other nasty stuff left over from combustion and forms acid in the oil which then corrodes the steel components. A good long run to get the oil hot evaporates this moisture from the oil.
As said above, a short run doesn't get the oil hot enough and just makes the problem worse.