Painting thin long lines in oil

​Hi

I cannot specifically state what the Old Masters for this purpose, as there is little archival documentation and organic analysis is far more complicated than was once believed. I would suggest that any additions to the paint were rather simple and not likely to contain high proportions of soft resins like mastic or the resin in larch turpentine. The reason that I say this is that fine thin lines applied over dry underlayers are the most susceptible to abrasion and yield under the actions of strong solvents and extreme pH. These were precisely the materials used by restorers in the early days. If it was common practice to add large amount of soft resin to the paint, there would be few examples of these fine lines on Old Master paintings for us to see today. The danger of this practice is obvious when examining 19th century maritime paintings where the rigging is often completely abraded away when the artist used a lot of resin to accomplish these fine lines and/or when a poor restorer cleaned the work without the knowledge and sensitivity needed to clean such works.
I would separate the methods of the Renaissance and Baroque from those of the 18th and 19th centuries when it was almost universally accepted practice to use mediums with very large amounts of soft resins. This is an instance where mediums containing hard resins and oils yielded more stable results than did the mediums containing softer resins (BTW I not stating that resins are necessary). The less soluble hard copal and amber mediums (if amber was ever a common paint additive is a completely different debate) create a paint film that is more resilient to the actions of cleaning. The oil-fused hard resin mediums did tend to yellow strongly and become brittle if used indiscriminately. Today we do have alkyd mediums, which fulfil the same role but are less yellowing and more flexible. Whether you like the way that they feel and move is another issue. I have generally preferred the fluid alkyd mediums to those that have a gel-like consistency or an intentional thixotropic quality in emulation of megilp.  
So as to practical application tips, I do think that some addition of a long oil like stand or sun thickened oil in a suitable amount of solvent added to the paint will help. The couch of oil will make this easier but the practice is problematic. This is the same issue as that of oiling out. Any oil film that is not covered by a layer of paint will eventually darken and become obvious. This may be minimal if the application is extremely thin, but even then it will probably become visible in the long run. You may be interested in browsing through the section on oiling out in “Resources Section” to read a bit more about this issue.
It seems to me that after the skill of the artist, the most important factor is the choice of brush. In my experience, the best are high quality kolinsky rounds with longer brush hairs. There is a reason that there is a brush made specifically for painting the super fine lines used to depict the rigging on ships. These very long rounds are even called riggers. Now these may be too long for your needs but you get the general idea.

Stand oil is not the best choice if you are trying to avoid leveling. Its inherent quality is to level. Linseed or Walnut oil sun thickened mediums, or to a lesser extent, artificially oxidized oil mediums, will be better for this as they tend to set or hold their shape, and set before leveling. Probably the best that I have found for this effect for a black or dark paint is thinning it with a dilute true copal-oil varnish (or possibly a true amber-oil varnish) as they both set quickly and resist the action of restoration solvents in the long run. I mention dark oil paint because both of these will darken substantially and dark pigments will be less effected by this change. I will probably get some push back on this as these materials are controversial in the conservation world for many reasons including brittleness, and yellowing. Used in this way and in fine isolated strokes as described there should be no preservation problems. The problem here is that true Congo, Zanzibar, and other hard copals (and true amber varnish) mediums are either unavailable or very expensive. Really, there is little benefit of these mediums over a high quality fluid alkyd medium for this purpose. I personally find the gelled alkyds less useful for this purpose.
To the 2nd question, 1:1 mixed with what? 1:1 oil (or even resinous media) to oil paint is way too fat for anything other than special effects, and probably too much regardless, unless you are willing to deal with the eventual strong yellowing  that will inevitably occur. 1:1 oil paint to 1:5 oil solvent may be perfect. It may also be too thin for certain FINAL effects. We need to maintain a bit of rationality in this and you should be able to come up with a sensible ratio.