4" Cast Iron usage with 1.6 flush toliet
We have a toiiet that is dated 1950 and still works but still flushes a 5 gallon amount. I would like to replace with a 1.6 flush using exiting 4" Cast Iron drain. There is about 2 to 3 feet of horizontal pipe. I was told by a person at the local plumbing store that I should not use the 1.6 flush because there is a small amount of water to wash the solids in the horizontal area. He said in due time it would cause problems due to backup.
Can you help on this matter?
Thanks in advance.