Ask DS: Steel Shot in Old Guns

[brake man]

My old 12 gauge may have bitten the dust so I may be hunting with my Dad's old Sweet 16 or 20 gauge Fox Model B SXS. The 16 is about 70 years old with an apparently original IC barrel. The SXS is close to the same age and has MOD and FC barrels. No screw in chokes for either gun. Dad is still alive but gave me these guns several years ago but said they needed to be hunted with and not sitting in a gun safe. I have typically hunted with bismuth or classic but that is with just a hunt or two a year. All that to ask if any gun experts on here can advise if I can/should use steel shot with these guns or if that will damage the guns. Any advice would be greatly appreciated.