You really want the 5902's, not the 5031's, for long life. These tubes are interchangeable, but the 5092 has the long lifetime. My first nixie clock design uses six 5092's and from the 4 clocks we built, we've had no failures or visible degradation after 5 years (that's over 1 million device hours).
I have a few 5031's, and several have deteriorated (partial or no glow on certain digits) and multiple attempts to rejuvenate them have accomplished nothing. Based on the coloring & behavior, I'm fairly certain the original 5031's (orange glow) are neon-only, and the 5092's (orange+violet glow) use a mixture (probably modified Penning w/ Ne+Ar+Kr) and Hg.