The Collins Cobuild Dictionary of Phrasal Verbs has a Particles Index. The introduction to the index makes it clear that in many cases the particle emphasises, modifies or extends the meaning of the same verb without the particle.
The index then lists the common meanings of each phrasal verb particle. It has 14 different meanings of the particle out.
For example, the main dictionary entry on help out states:
If you help out or help someone out, you do them a favour, such as
lending them money or doing some of their work.
The particles index lists help out under Section 13: Supporting and helping, along with bail out and several others.
Write out, print out, type out and clean out are in Section 8: Thoroughness and completeness, with many others such as carry out and hear out. The entry to this section states:
You use out in combination with verbs which describe activities in
order to indicate that the activity is done thoroughly or completely.
Test out and list out do not appear in the dictionary but could be included alongside the others in Section 8.
Skip out is in Section 1: Leaving, which includes the introductory text "leaving a work activity".
Separate out is in Section 11 Arranging, dividing, selecting, and distributing. Also in this section is rent out. But rent and rent out have opposite meanings, at least in my variant of British English.
So, in summary, it is not always true that particles "add no additional meaning".