I visited Venezuela during the 70s starting in Caracas and it was Poverty Ridden during the Oil Boom, not because of Marxism, but because of Ultra Bad Capitalism which eventually led to Marxists taking control like Hugo Chavez.
Bad Capitalism Breeds Even Worst Marxism.
During the 1960s and 1970s, Venezuela was governed under a stable democratic system established after the 1958 overthrow of dictator Marcos Pérez Jiménez. This era, known as the Punto Fijo democracy (named after the 1958 Punto Fijo Pact), featured power-sharing and alternation between the two dominant political parties: Acción Democrática (AD, Democratic Action), a center-left social democratic party, and COPEI (Social Christian Party), a center-right Christian democratic party.
This form of government occurred during the Oil Boom where gains were concentrated and the masses increasingly suffered.