For example, a Spanish citizen with his 1971 S.E.A.T. 124 (Spanish-made fiat 124), is travelling from Spain to Mongolia via France, Italy, Slovenia, Croatia, Serbia, Bulgaria, Turkey, Georgia, Azerbaijan (crossing to Central Asia is made by ferry across the Caspian Sea from the Port of Baku), then Kazakhstan, Russia and finally via the Russian/Mongolian border crossing to Ulaanbaatar. The car has Spanish registration number plates and renewed vehicle registration certificate.
So, what documents of the car required at the Russian/Mongolian border crossing alongside the Spanish passport? Does Mongolia recognize carnet de passage? Are border insurance required at the Mongolian customs?