Oil painting mediums

Hi, This is a multipart question so it will be easier for me to answer the separate sub-questions within the body of your text (in red)
Hello! I have a few questions regarding oil mediums.
1. I have done some of my paintings with oil medium that has Venetian turpentine in it. The formulation of this medium is one part of stand oil, one part of Venetian turpentine (Schmincke Natural balsam/larch turpentine) and two parts of turpentine ( Lefranc & Bourgeois rectified turpentine) . I have read that Venetian Turp/Balsam is not that recommended, but at the time i didnt know, so i did some very important paintings with this medium. What are your thoughts about this medium? I was very carefull about layers ( fat over lean) Do you think that these paintings will be ok, even if i did them with this medium? 
Venetian turpentine is a natural balsam, meaning that it is a soft resin that still contains the essential oil. Like all soft resins, the resinous component will remain sensitive to solvents. Oil paints containing soft resins will also be more sensitive to solvents. If these works are varnished and the varnish ever needs to be replaced the oil paint could be damaged in the process. If the work remains unvarnished this is less likely to be a problem although that will cause lead to other issues and my not be the aesthic that you are looking for. If the work is varnished the possibility of a problem will be direct proportion to the amount of medium that was added to the paint. Small amounts are unlikely to have a major effect, larger amounts are much more likely to cause problems if the painting is ever cleaned. Fat over lean refers to the idea that subsequent paint layers should be more flexible than those below them. This would have no impact on the solubility of paint layers other than the likelihood that your last paint layers are likely more sensitive to solvents than your first.
2. I was also using Winsor and Newton Liquin Original medium.
What are your thoughts about this medium, do you think this is good archival medium to use?
Alkyd mediums like Liquin are stable and when added to paint do not increase its sensitivity to solvents. Whether they move and handle in a manner that is pleasing to you is a separate issue. I do worry about people loading their paint with excessive amounts of alkyd mediums because they believe that they are not deleterious. Alkyd mediums do yellow, like almost every organic material. Excessive additions of alkyd will result in a paint film that will amber slightly over time.
And is it really true that you dont have to be that careful about “fat over lean” rule with W&N Liquin original? I always try to be careful about layers even with this medium. 
I have never heard this and do not agree. Recent studies have shown that alkyd paints are slightly less flexible than oil paints containing the same pigments/etc. While the addition of medium to an oil paint is not exactly the same as an artists alkyd paint, there are similarities that suggest to me that this concept is flawed. I am not trying to contradict myself here, I do believe that alkyd mediums are stable, just not that their use allows the artist to ignore sensible working methods.
I wanted to ask is there some substance that can be added to Liquin as a thinner, some solvent, because i paint in several layers? Or i should just use less Liquin medium in first layer and then add medium as i build up the painting? 
I find that when people use straight alkyd throughouttheir painting it is very easy for the painting to become excessively greasy if there are many layers all containing neat medium. Of course, each artists working method will be different but it seem to me that it would be sensible to use no medium on the first layer or so, a bit of medium thinned with a solvent (like mineral spirits) and straight medium added to the paint only for the final layer(s).
Also i have combined two Liquin mediums on one painting. 
I used Liquin original for first layers and then used Liquin Fine Detail for finishing layer. I have contacted W&N about this and they have told me that is totally ok to combine those two mediums, i just have to keep in mind “fat over lean” rule when i am combining them? What are your thought about this? 
This sounds reasonable and correct.
3. In museums there are some paintings that stood the test of time, and these artists did scumbling techniques, and glazes, and it really seems that they sometimes didnt care that much about “fat over lean” rule, and still it looks great. Do you think that we should maybe try to some extent to apply these rules, but not to be very precise and that concerned about it?  
To be honest, I know of no documented use of the term “fat over lean” or even an allusion to a similar concept before the 19th century. It is likely that it was just natural to construct a painting in the manner that roughly follows a similar procedure. Dryish paint does not adhere well to greasy paint. Glazes tend to work best as the final couple of layers. Scumbles were certainly used but these can be applied lean through physical manipulation and by the addition of additional medium. Follow sensible technique but do not be so imprisoned by them that you are unable to make the artwork that you want to express.
4. And i wanted to ask what types of oil mediums were least changed through time, and prooved to conservators/restorators as most stable? 
Generally, mediums that contain only oil and solvent or at least, those that do not contain a soft resin are the most stable. Soft natural resins promote solubility, yellowing, and brittleness. Hard natural resins promote brittleness and yellowing but at least they do not promote solvent sensitivity. This is practically moot as there are no readily available hard resin mediums (outside of the boutique materials, cottage industry art materials market). Oil only promotes yellowing and only if used in excessive amounts.
Alkyd mediums are a good middle ground as they create a paint film that is resilient like an oil only medium but contributes the gloss.and refractive index of a medium containing resins (to be clear, the alky is an oil modified synthetic resin).
I have heard that these were the ones which were more basic (stand oil + turpentine, or white spirit, or something like that). 
What are your  advices about medium choice and what is the best way to  paint in layers? 
Simple formulations and layering systems are generally better than complex ones. Work in only as many layers as is necessary to create your intended aesthetic effect. Do not overload you paint film with mediums and reserve their use for special effects and for later paint layers. Follow some sort of system where you mostly apply nmore flexible layers over more rigid ones. You can do this with judicious additions of a stand oil/solvent medium or using an alkyd medium. For some paint effects, you can do this without adding any medium.
Thank you for time and efforts!
I hope this was of some help.
All the best!
 
Marko Karadjinovic 

I can see no harm in adding solvent to the alkyd medium, especially for lower layers. It may move differently that the unthinned medium but it should not be less stable.
There is nothing wrong with the medium(s) you propose here.
The medium of the van Eyck brothers has fascinated and caused arguments for over a century. Chemical analysis has found nothing unusual. Most today believe that his medium was simply a drying oil that may or may not have been bodied and perhaps a small additions of a resin to increase the refractive index and deepen color saturation. The problem with this is that we now know that it is very difficult to analyze paint samples many hundreds of years old, even more so when the works have been restored over many years.