Thanks for the links but they didn't help me. I have various custom filters from Adguard and probably they would have to be somehow converted to work in Opera. Unfortunately, as also stated in the link to another thread on Opera forum, there are no information, instructions or whatsoever how to do it. Opera should compile a manual!
"There is virtually no documentation about this anywhere. Not even in the release notes! It would be great if a manual for the general syntax is specified online somewhere for advanced users. Users who are unaware might just port their ABP custom filters, hoping that they would work. An online conversion facility to transform filters in other syntax to Opera syntax would also be useful."
Have no other choice than to run my custom filters in Adguard. Shame.