suppose somebody invented a cheap chemical additive to induce a 40% fuel saving, what should he do?
Probably nothing considering it wouldn't save anyone any money. A little suspicious that supposedly oil prices are so high because of its scarcity. However, ethanol which is usually cheaper than regular gas is now more expensive. Diesel which is a less refined version of gasoline is also more expensive than the oh so valuable gasoline. Make much sense? I think not.
If you patent it, it puts the technology in the Public Domain. This means it can be modified and used by others. Unless you are very rich you wouldn't be able to fight it. Check very thoroughly that it doesn't exist already. Eg. search fuel additives aluminium and so on. If you really have something special then use an intermediary Lawyer who will act for you, keep your name secrect, get the interested parties to sign non disclosure agreements and so on.
If you believe the various scifi stories in that vein, change your identity and hide before the oil corporations find you. Besides, all you would accomplish by sharing the secret is delaying the coming collapse of the oil economy (and the planet's civilization), which would worsen global warming and increase the number of people who would eventually starve to death.
