I only know that x and o stay the same
SIS
this \
SOS
SWIMS
The word SWIMS, when written in upper case letters, is the longest word in the English language that reads the same both forward and upside-down with no reading difficulty.
The word is NOON.
In a direct flip, only N, X, H, O and I are the same upside down. In a flip and rotate, S, E and D are also the same and, depending on the type of font, B and K may also be the same.
NOON
noon
Capital letters which read the same normally and upside down are: H, I, N, O, S, X, and Z. Words that can be made from those to satisfy your criteria might include: NOON NON SIS (slang) SOS (acronym)
There is more than just 1. C, D, O, S, and B are all curved letters that are the same upside down.
When you use capital letters, SWIMS can be read the same upside down. Note: Nabisco had a snack mix called 'doodads' (now discontinued), which they spelled on the box in all lower case letters. If you placed the box upside down in the cupboard, it read 'spapoop' so our family started calling them 'spapoops'. That was many, many years ago, and still today, I had to stop and figure out what that real name was.