Depends on the car… My 92 buick that’s supercharger with an ancient obd1 computer system with no knock sensor? I’ll mix 93/89 to get 91 but I wouldn’t risk anything lower since I don’t wanna crack a piston. Modern cars? High comp turbo engines can run 87 no issue.

You looking for rotary companies? There’s tons

DO NOT RUBBER UNDERCOAT!!! You’ll actually make the frame rust worse!!! Get a fluid film (oil based) undercoating instead please! Needs to be reapplied/inspected once per year!