Gold is pretty much the “standard”. Like a 20-dollar bill. Well, when you’re a kagillionaire, and you’ve hit the jackpot after a nice raid, that is a lot of change to carry around in a land in which banks are pretty much as corrupt as the corner cutpurse.
It’s easier to carry around platinum. Or rubies. Or pearls. Or hell, oyster shells. Whatever is rare in the land your DM has created. I think we did basilisk teeth once. There was a nest nearby, and the basilisks used to run off with the city’s children, so anyone who killed a basilisk could get the world for a basilisk tooth. Eventually trade in basilisk teeth becomes a commodity. 25 gp per tooth. Nice and light.
We didn’t much worry about encumberance, though, as long as you kept it reasonable. Obviously a Human Fighter with a STR of 18/90 could carry quite a bit more than a halfling magic user with a STR of 5.