Coolant

My 15-year old Civic's coolant in the white plastic resverve tank is at the bottom but the coolant in the radiator is full. Have not had coolant leaking issue for many year after I bought it 2nd hand. Checked the onwer's manual which says only Honda coolant should be added to the reserve tank to avoid corrosion damage to the alumminum engine. My question: is generic good quality commercial coolant ok for Civic, or Honda coolant is a must? Thanks