Rear pre-load adjuster trouble

So I was going to adjust the pre-load on the rear shock today. Just enough to soften it a bit. But, when I tried to adjust it I found it was seized and started to damage the screw head.
Grrrr. Stop and spray with penetrating oil and wait till another day.
Any suggestions here. The rear feels OK, but I would like it a bit softer.

 

Maybe it is already as soft as it can go? Will it turn the other direction?

 

It will not turn either way.

Sent from my Moto Z (2) using Tapatalk