Some will say do not mix ATF with hydraulic fluid, but ATF is hydraulic fluid with some additives and dye. Years ago, when the recommendation came out to use CAT stiction fluid or fork oil to stop the popping, I just drained some fluid from the reservoir and added the CAT fluid. Some years later, due to the original recommendation to drain and fill the fluid, I just drained what I could from the reservoir and replaced it with CAT fluid and ATF. I have never had any issues with the hydraulics with the mixed fluid. This is the link for adding stiction fluid to the system:
https://lci-support-doc.s3.amazonaws...cd_0002091.pdf