Kerosene, then ATF, then Hoppe's #9 gun solvent is the order of things for those machines that are so dirty with caked on oil and debris that sewing machine oil won't do it.
I've never had one that was so bad in the upper arm that I had to take it apart, but I have removed all the inspection plates and cleaned as much as I could reach, then flood it with sewing machine oil to flush out the rest.\
Joe