Cable replacement. May I use speaker wire to replace the present wire in my garage door opener?

The one thing about those cables is the waste number of cords, make them way more prone to short on the terminal (because the one thin that you didn't saw) - screw terminals are the worst in my opinion.

So, I'd say the answer really depends on the kind of terminal you put the cable into.

You may solve this by adding a wire terminal clamp or wire ferrule at the ends of the cable to mitigate potential future connection issues to the cable.

This assume low voltage not exceeding the cable voltage rating.

If you have AWG20 stranded speaker wire pair for each line it will be the equivalent resistance per unit length as AWG 16 solid copper with PVC jacket rated for some x kV insulation rating. AWG resistance runs in powers of 2 resistance every 4 guages. But then you need a ground wire.

Does it meet code requirements in your region, probably not. Will that work ? probably.