That would be true if the piston were in a free state in the oil, but it's not. It's bounded very closely by the cylinder on all sides, creating a small gap through which the oil flows. The piston cannot get up enough speed for the taper to have any effect, since the oil flow around the sides, and through the holes, will go turbulent long before the hydrodynamics of the piston shape come into play.
I'm not saying it doesn't make a difference, I'd just like to know why!!
