It's a good idea to always drop the oil whenever you do any kind of work involving coolant, it's going to get in your oil no matter what kind of precautions you take.
Drop the oil and filter, sounds to me that the filter media has collasped due to the thick oil.
Are you using fram filter?
Switch to a Napa, Wix or Purolator pure one filter and you wont have that problem any more.
Goodluck, Cp