15lb cap will be fine, the difference between recovery and non recovery is the recovery caps have a one way valve/flap and top seal so when the engine cools down it allows the coolant to be sucked back into the cooling system. Any leak around the cap will allow air to be sucked in and not coolant from the overflow bottle. When the system pressure exceeds more the 15lbs the cap pressure spring allows coolant to vent to the overflow bottle. If the system is working properly the radiator should be 100% full at all times.