Gasket sealant.
ive never had a problem with silicone infact the opposite having never had a base or head leak adam has it bang on its a seal incase microscopic amounts of burnt air try and escape, as for not being petrol proof its not like you are running neat fuel over it continuously its a very fine smear of the stuff
We sleep soundly in our beds because rough men stand ready in the night to visit violence on those who would do us harm