The MSDS for the current version of 3 in 1 oil shows it's about 98% severely hydrotreated naphthenic oil and 2% naphtha.
Hydrotreating came along in the 1950's, it appears the product has evolved some since it's introduction.
Hydrotreating saturates the hydrocarbons with Hydrogen making the oil more stable.
Naphthenic oil is a better solvent than paraffinic oil, contains less wax to be removed during refinement, and can be more easily made to a low viscosity finished oil.
3 in 1 would appear to be a standard, conventional, light oil with a tad of solvent (petroleum naphtha aka lighter fluid).
It's a general purpose light oil. It does not say 'gun oil' on the label but it should be so close to any 'conventional' (vs. synthetic) product that is labeled 'gun oil' as to be indistinguishable in service.
