1971 Ford F-250 help!

Hello all! I have a 1971 Ford F-250 which has a 390.

I haven’t had this vehicle long, and while attempting to jump it to get it home a few weeks back, a ton of snow fell into the engine bay as I opened the hood, the heads were also iced over (below zero cold snap all month) I drove it to my mechanics shop, never overheated, neversounded weird or anything.

It needed a new rear main seal. As it would leak severely after driving, a couple courts almost. But upon inspecting it after getting it to its destination I noticed milky substance under the crankcase cap and on the dipstick, obviously a head gasket symptom correct? Well before we proceeded with the seal replacement we had the cooling system pressure tested, absolutely no coolant was escaping, the cooling system pressurized just fine and we left it overnight to be sure and the next morning it hadn’t dipped down in pressure at all. So I guess I’m looking for what else it could be? I mentioned the snow cause we thought it could be just excess moisture from the cold and elements, but we also didn’t rule out the possibility of an additive being put into the crankcase, like stop leak or what not (the guy I got it from was not exactly bright)

The head gasket a were ultimately determined to be totally fine so, any other guesses? The rear main is fixed and when I go pick I up and road test it, I’ll be able to see if the problem persists or not.