By work i mean if they give any effect by adding those additives that supposed to reduce friction and give better mpg, or is it a waste of money...
That's impossible to answer by anyone for 2 reasons ( by answer I mean in a method validated and vetted by a legitimate scientific method tenet)
1- Most additive packages ( bottled entities) are simply blended to pass a
TEST and not provide an actual performance standard. This is how they are legally protected to a degree AND they already know an individual engine is an "infinite unknown set of variables" and nobody can make a product that "guarantees" to do a "thing" for every scenario. ( that's simply impossible)
Manufacturers know the "standard tests" are designed to measure
QUALITY AND CONSISTENCY so products can be evaluated to a common standard horizontally and vertically.
RARELY are the tests
directly representative of actual conditions encountered in the field. To make it even worse, it is often that a test is designed to isolate a specific part at the expense of other parts or define a parameter ( such as temperature) that may or may not be the same in the field.
These are reasons that we often have to design custom tests ( that match real world conditions) and routinely find that the "published" advertising points are meaningless and sometimes "lesser rated items" actually perform better in the real world. ( or have to create a special blend)
2- Even if you knew all about 1, its a virtual impossibility to separate and segregate the additional performance of "product X" from the base oil in use. Even under laboratory conditions, this would be almost impossible with a huge range of error and cost tons of money and have to test enough engines to have a good data set and nobody is going to do this.
Snake oil manufacturers know this too and will use it to promote their products and to hide behind because you "cant prove it doesn't" either.
Summary- "waste of money" is subjective and in the eye of the beholder and there is something to be said about "peace of mind" but know its a false sense of security and confidence because its based on a belief solely based on anecdotal evidence and only verified by confirmation bias and suspension of disbelief.