Avgas typically has a lower specific gravity than car fuel. This means you need to tune the fuel flow to flow more to get the correct A/F ratio. In other words, if you use avgas and don't tune for it, you could run too lean.

 

Yeah I listened to all that for a long time too. Son-in-Law Josh (pilot) has been running Avgas in their 944 Turbo race car for years. Always with an AFR meter and no problems.

I recently switched to PMOs and installed a meter and it ran perfectly last event on 100LL.
Last tankfull I put in 98 VP to compare but never made it back out on the track.