I've been thinking about what could be the cause of this. The only thing I can come up with is the blocker doesn't seem to clear the oil away from the cone face as well as it should. With light shift pressure I'm guessing there is still a fluid pad/wedge between the blocker and the cone, which is allowing the gear to continue to rotate. That would cause the occasional grind/crunch going into gear.
With higher shift pressure, it forces the fluid out more aggressively, and allows the blocker to fully engage the gear. I'm also guessing that once the blocker and cone faces wear in a little, you pick up some contact surface by wearing down the high spots on the friction material, which improves the situation.
Different fluids, particularly synthetics, likely have different cling and shear properties. That would also affect how pronounced this is, as a better flowing fluid would clear off of the cone better, and allow the blocker to engage the gear. That would explain why some people claim changing fluids fixed their issue, or made their transmission shift better.
This is all speculation on my part, but it seems plausible based on my limited knowledge of how it works. Unfortunately, it doesn't do anything to fix the problem.
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