Oh dear god, this reminds me of the good old days of the first Look cleats and then the PD-6401 pedals.

They would creak incessantly after the cleats had a few hundred miles on them. The creak comes from the contact point between the back edge of the cleat and the clamping part of the pedal.

There were two ways to fix this, you have discovered the first which is to use some kind of lubricant. Pledge also works pretty well.

The second thing to try is to lightly sand the slick back edge of the plastic cleat just enough to take the polished/shiny look away. If you look at the back of the cleat, you will see where the contact point is polished smooth by the pedal clamp. Just lightly sand it with 400 grit sandpaper.

Repeat as necessary, neither are permanent.

I switched to SPD-SL a year or so after they came out. Pedal creaks are a thing of the past.