The idea that oil is in the coolant is (and I don't to say this) a crack in the cylinder head even though they are remanufactured, and pressure tested. I do recall the 1991-1998 SOHCs cylinder heads had a casting defect, which caused them to crack near or on the camshaft journals. That allowed oil to mix with the coolant. Other than that, the engine block could have cracks in tile oil and engine coolant passages in the cylinder liner area. That will need to be inspected if the cylinder head checks good.
The best way to check this is to remove the camshaft and then pressurize the cooling system. While pressurized, check for evidence of coolant at the camshaft journals.
Keep in mind, a remanufactured head was used. Many of these heads had problems that led to them needing replacement. If I had to guess, I suspect there is a crack in one of the journals. Also, I don't believe they pressure test the cooling system passages. That is where the leak would be found.
Let me know if you have questions.
I just found the technical service bulletin. I attached it in the pics below. It describes what I mentioned and how it's fixed. It also provides the procedure for properly flushing the system. Let me know your thoughts or if you have questions.
Images (Click to enlarge)
Thursday, October 21st, 2021 AT 8:17 PM