[nUcLeArEnVoY]

Motor rebuilt at any point? If it's an original motor, it's not unreasonable to figure maybe the original head gaskets finally just gave way or the water openings corroded out into a compression or oiling opening. You run antifreeze or straight water? Corrosion protection in a cooling system is almost as important as the cooling capacity of the fluid itself.

It is a rather odd thing to happen out of nowhere. I'd do further testing to diagnose a head gasket such as compression test, the suggestions above, or maybe the CO2 fluid testing kit, though that would involve running the engine while it's potentially vulnerable. Don't forget that with these engines, there is an entire other part of the motor where coolant has an opportunity to mix with oil, and that's the timing cover - those openings are only separated by a crapola paper gasket.