Water in the oil is not good, but antifreeze in the oil = a dead motor very quickly.
I assume you are pressure testing your cooling system while its still cold and have found a minor leak. Once the motor starts heating up, about 1/2 way to operating temp, I've seen a lot of coolant leaks get worse, but then they close up when the motor gets up to temp.
If you are unsure if your coolant leak is block related, drain the antifreeze out of it and fill it with water until the block has been sealed again. Gene