I know better than to get in a discussion when the experts have spoken, but from what I've gathered, Tru-Oil has been used effectively on acoustics because it does not penetrate really hard woods. Softer woods, though, require a sealer before the Tru-Oil is applied. At least that is my understanding.
Whether it fits Bruce's definition of an oil varnish or not, I do not know. Perhaps Bruce can chime in. However,
Luthier's Mercantile International describes it this way:
Most oils available as wood finishes are either linseed oil or tung oil in some form, with or without any additives. Among all of the oil varnishes available for use as a simple wipe-on finish for musical instruments we offer and support the use of the Tru-OilŪ product line. Tru-OilŪ is a polymerized linseed oil with other natural oils added. This formulation will actually build-up as a finish unlike the raw or boiled linseed oils. The Tru-OilŪ wipe-on varnish (our part FTO) and its matching Sealer-Filler (FTOS) have been used for years to finish hardwood gun stocks so its properties and application process are well known. Even though it is not as protective and durable as a lacquer finish it produces a hard, thin and flexible finish that has no detrimental effect on the tone of the instrument.
Regards,
Bill