Are you filling the stuffing tube? I do 4-5 (A hydro) to 8 (Thunderboat) pumps on my grease gun up the stuffing tube from the strut before pushing the cable through, then slowly rotate the cable in so the grease doesn’t just push out the other end.
I’ve been using this stuff for a few years now and never have a problem. Load up the stuffing tube before a race day, pull out the shaft and wipe it clean after the last run.
I got it at West Marine.
I have been greasing and installing my flex shafts the night before a race, and taking them out and cleaning them off after the final run for 6 season on up to 6 boats at a time and never once replaced a shaft due to wear or breakage. [knock on wood]