There is nothing in ARX that will cause harm to your engine. It will however help bring your 240Z back to life. It will help restore compression by cleaning up the rings, and allow your rotating seals to regain their elasticity. If you think it is running fine now just wait until ARX has had a chance to do it's magic.
Please replace the old oil with new before any driving, 20 years have certainly degraded the old oil so much it is about useless and can damage your engine. Run the car with new oil (mineral) a few miles, then start ARX application per instructions.
Please realize that if any of your seals are cracked or ripped that Auto-Rx cannot repair them and you may need to replace them, hopefully this is not the case. I have no idea what can happen to the seals on an engine that have been sitting for 20 years.
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