I don't think I would mix the two. The XL-18 was designed to compliment the QDC from Ford. Being that I'm no chemist, I don't think I would leave it to chance that mixing XL-18 with anything but QDC could lead to disastrous results. Even if there was a difference with the immediate results, who's to say that the XL-18 wouldn't cause premature breakdown in other fluids and cause increased wear within the tranny?
I do agree. Seeing if anyone has already done it before I try but kinda thinking NO. I have another post asking what fluid to change to (Redline MTL, Amsoil, or Royal purple) but only 1 reply for Amsoil so far.... should have done a poll I guess. I have read where MTL works great but Redline (& Amsoil) pulled the spec out for our transmissions recently and wanted to see if RP not only meets the spec but has same improved results.