Automotive oils are not the same formulation at all as aircraft oils. Use the oil designed for your powerplant, period.
You should never mix brands or designations of turbine oils...ever. You shouldn't mix any piston oils with turbine oil. You shouldn't mix automotive oils with aviation oils. You shouldn't use oils designed for one application in a different application, mixed or not.
Automotive fuel and automotive oil composition is not uniform. While ASTM/SAE and other standards exist, chemical consistency isn't the same batch to batch, brand to brand, etc. Avaition oils and fuels are very closely standardized and regulated, and the quality control much tighter. Formulations for a given automotive oil spec vary with the manufacturer, going so far as to have different base oils, numerous different additives, etc.
Aviation oils do not. In the US, the American Petroleum Instritute provides given standards for oil, but allows a wide variety of formulations to meet those standards...and there's no way of determining if the auto oils meet aviation standards (particularly with the variety of formulations and lack of testing).
Aircraft oils stay consistant in formulation and standard. Automotive oils change in formulation constantly, and there is no consistancy.
Why not use automotive oil in your aircraft piston engine? If you're going there, why not just fill it with hydraulic fluid, cooking oil, turbine oil, or oil of oly? Use the right product for the right job. Does anybody remember the Phillips XC debacle about twenty years ago? Or the recent Mobil 1 debacle? Wonder what happens when even small changes are made to the formulation of the oil? Ask the oil companies that shelled out very large sums of compensatory funds for the damages caused.
As for someone who mentioned using Slick 50...do NOT do that. Slick 50 is responsible for damaging far more engines than it's every helped, and it's never helped any. Folks think it does, but it doesn't. It claims bonding of PTFE to the metal parts, but engines never reach a temperature where this is possible. Instead, all the junk in the bottle stays in the oil, occasionally pluggnig oil galleys, filters, etc. Wheather the engine is making metal or not is irrelevant. The US army used slick 50, and forbit it's use after the amount of damage it can cause came to light. So did a lot of other operators. That it's still on the market today is an absolue mystery to me. I would never put it in my car, let alone an aircraft.