...I have an Apple Leather case for my iPhone 8 Plus - ok, its not a X but its the same quality case and mine is not slippery at all.
I've only had it about a week (got the Cosmos Blue one) and it actually naturally grips my fingers, when I hold my phone. Perhaps it's the way you are holding your phone? I always see people holding their iPhones with a pinky finger underneath the home button area, in order to support the phone but I have to say, I have never held any of my iPhones in this way...!
I always hold the phone in my right hand, right side of phone rests in the crook of my thumb, with the right hand bottom corner of the phone, resting within the fleshy part (base of right thumb) of my hand. I then support the phone with ALL 4 fingers on the back of the phone - this is far more comfortable than having 3 fingers on the back and the pinky finger trying to support the phone from the bottom.