If you have five or more mana values among cards in your graveyard and then some of them are removed such that you have fewer than five mana values among those cards, Syndicate Infiltrator loses its bonus to power and toughness. Notably, this may cause it to die if it now has damage marked on it greater than its toughness.

“Five or more mana values among cards in your graveyard” means there are at least five different mana values among those cards.

The mana value of a split card in the graveyard is the total mana value of both halves of that card. It does not have two mana values.

The mana value of a double-faced card in your graveyard is always the mana value of the front face.

X is 0 when determining the mana value of a card in your graveyard.

The mana value of a land card is 0.