Don’t use gas as a mix. Use diesel. Better for the environment. Then spray the rusty bits on the car. The degradation factor of the oil is the issue. Newer oils at any grade last longer due to new mixes. If you have a oil burner you can mix in a pint or so per gallon. I just take my oil to a guy who has a junk yard with a multi fuel burner and he filters and mixes the used oil for his shop.