Tipping after paying is normal. Tipping in foreign currency is not. If it’s that large of a tip, and you explain the conversion rate, I imagine most people would be thrilled. But if an unknown or average amount in OMR is the only tip you give at a restaurant, their first reaction may be anger, as they will be expecting an approximately 20% tip in US dollars.
The term was original essentially a synonym for anarchist, being coined by Joseph Déjacque. Modern American libertarianism broke from its roots and promotes very un-anarchist perspectives. However, in my observation they do use anarchist framing and terminology. I view it as a right wing ideology for people who are attracted to the left. I believe, for this reason, libertarians are some of the easiest leftist converts.
In case anyone is interested, I made some videos to help explain both Kropotkin and Bookchin: