I expected that by applying large positive voltage to gate, I would increase gate threshold voltage. But as @RoGeorge accidentally found in #Your MOSFET is not good enough? Then modify it!, positive torture voltage leads to the same direction of gate threshold voltage change. IT GOES DOWN! And I can confirm it!

Does it make any sense to you?

-42V ->Vgt decreases

+42V -> Vgt decreases

whaaat??!

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Because the effect is very sensitive to the voltage (39 V -> it takes hours, 42V -> seconds), and because in positive torture voltage I can't monitor transistor shifting in real time with my blink torture circuit, I decided to go for current-mode torturing. From how brightly the LEDs in series with torture voltage source lighted up, I deduced the gate current was about 2 uA when I applied -42 V. So, for positive voltage torture, I connected torture voltage source through a 10M resistor, and cranked it up to 60 V. Works!

This leads me to another experiment. Maybe a current mode torture is going to be more linear. That's what I'm up to now, with the only problem that I don't have a controllable microamp-range current source. So I'll need to build one first.

One of my multimeters can measure down to 0.01 microamps, so seems like a 10M resistor in series with adjustable voltage power supply will do. I have to think about it a bit.