Also, if your gasoline has sulphur in it (it does), this sulphur can react with water and oxygen to make sulphuric acid. This is some stuff that is seriously bad for your engine. Your oil has special ingredients in it called buffers to neutralize acids. Finally, your engine can get internal build ups of tars, waxes, and other gunk. Your oil has solvents to try to dissolve this stuff and get and keep your engine clean.
Very little sulfur really, regulated to tens of part per million levels, and most of it is burnt in combustion to sulfur oxides that will enter that atmosphere, combine with water, and become 'acid rain'. Little to none passes the piston already reacted with water to sulfuric acid. Most (or all) oils DO NOT have pH buffers, as a pH buffer relies upon an equilibrium between two ionic salts in aqueous solution.
Those tar/wax build ups are break down products from the oils.... Any solvent for these genuinely added would damage the lubrication and temperature/viscosity requirements. The 'solvents' spoken of often are the result of carbon chain breakdown in the engine to lower MW compounds. i.e. a C13 chain cracking to a C8 and a C5 chain. Similar to hydrocracking done to convert long chain heavy fractions to a more gasoline rich feed stream.
I've been through 3-4 refineries, for interviews, process review, plant tour or similar. (Shreveport is a bad neighborhood
) It's actually a pretty crude process, and not as sophisticated as the writer believes. There's not really 300 magic voodoo dusts added to motor oil, or any oil.