Automotive grade isn't "stronger" it's mixed with sulfur to stop people from inhaling it...
You can get stoned (and most people do) off of food grade nitrous.... even though most of those even contain all sorts of contaminants from the bottling/manufacturing process...
here's a quote from this site:
http://www.fiestaturbo.com/content.php?page=nitrous
about automotive grade with added sulpher dioxide to prevent abuse...
""You can inhale it and it makes you laugh"
Wrong - Although medical grade nitrous has this effect, automotive grade contains a minor amount of sulpher dioxide, an additive to the gas to prevent substance abuse. sulpher dioxide gas is released in the process of biological decay, something you don't really want to be inhaling! This additive does not affect performance. "