Sometimes 90's can't be avoided, such as the 90 fitting feeding the fuel log on wtm442's small block. But he is right, as a rule the more 90's in line the more flow restriction realized.
A single 90 in a low pressure system conducting a fluid with the viscosity of gasoline is of no consequence. Now if you are pumping
oil at 100 psi that changes everything, I was careful not to include any 90 degree fittings when designing my
oil circulating/cooling system.