Ehh..yes and no... AN is measured in 16s... so 6AN is .375 5/16 is .3125..thats a pretty darn close. It would basically be 5AN if it existed.
Technically...you're not supposed to go any smaller on the return side as to not cause any restrictions. But as long as you used fittings that didn't choke down that 5/16 any smaller. You'll be fine. Personally a roll of -6AN hose is all of $40-60 pending the kind you buy..even cheaper for a roll of tube....so i'd probably just do it right, but i understand using what you have.
ProClassic Hose Russell 632073 - Pace Performance Parts