So here's the facts as I know them:

-1959 Apache half-ton with a 235ci
-I popped the cap on my radiator yesterday and there is oil mixed with the coolant. Kind of milky/cloudy.
-It has been hot here so I have been checking my coolant regularly. There was no oil in the radiator as of a week ago or so.
-My temperature has stayed at a normal operating level.
-I checked and there is no water present in my oil.
-My last oil change was about 4 weeks ago and there was no sign of water.
-I have noticed a faint popping sound upon deceleration the past week or so.

I did some searches on this site this morning. I saw a couple of issues with cracking on/near the oil gallery that sounded similar to what I'm seeing. Does anyone have any experience with something like this? I've heard of water getting into oil, but not the other way around. Thanks.