As far as I know, Truoil is a polymerized oil/resin varnish, much like the other products discussed here. These thinned light varnishes, sometimes described as danish oils, are usually combinations of either polymerized linseed or tung oil, and resins. The resins can be alkyd, phenolic, or urethane. Presumably combinations of all of these would be possible, but I think usually either linseed or tung oil and one of the resins would be used. I think Truoil is linseed based, but I have no direct knowledge. For my purposes, it is too thick, builds up too much, and dries too quickly to handle properly in the finishing process.
These products, depending on their formulation, can have very different handling qualities and final results, and therein lies the rub (sorry). Remember, the finishes we're talking about here (I think), are more in the wood than on it, with very little surface buildup.
Incidentally, polymerized oil is oil (tung or linseed) that has been heated in an oxygen free environment to somewhere around 500F, which apparently modifies its molecular structure significantly for the better. Commercial "boiled" linseed is nothing but raw linseed to which metallic dryers have been added, and is an inferior product for any wood finishing purpose.