Oil seeping into coolant

[Elbrus]

I recently rebuilt a JD 48 A (engine resleaved, a different head (old one did not pass leak check), lead washers used on studs. Engine was run for about 10 minutes in the fall; water drained because I have not added antifreeze and tractor was stored indoors but still potential for freezing. This spring I can detect a tiny amount of oil around the water plug opening. Not enough to even drip. Engine has not been re-tourqed as it has not been run enough to get hot. For the month or two that water was in there was no detectable loss. Had I left it in over the winter (minus the possible freezing effect I"m sure this would have been undetectable. This is a small amount and if I had never drained the water I"m sure it would not have been detected. Is this at all normal?