I am looking forward to buying Geneva oils when they become available again in the UK, as I like that they are fluid. In the meantime, I have been premixing my own using the slow dry recipe.
However, I have read that use of medium should be kept to a minimum, as tubed paint is usually at an optimal pigment-to-oil ratio that when deviated too much from, increases the chance of problems such as cracking, wrinkling or yellowing. For example see the section called 'Use as Little Medium as Possible' here:
I personally don't like how thick oil paint is out of the tube. Even more fluid brands like Rembrandt are too tacky.
I like paint that flows off the brush easily, hence why I mostly use acrylics (which by the way I find is a gorgeous medium). And this is also why I really like the look of Geneva oils.
But I'd value the opinion of more experienced artists on the issue I discussed above - thanks!