I don't have joint trouble thankfully. I attribute some of it to years of ballet training. Not full on toe shoes craziness but streching and yoga type movement. However:

"I can't use normal butter. It spoils before I can get it used up"

I'm of the mind that the less chemicals foodwise the better. I think it's better to use a little butter than the chemical substitute. I must admit I love my butter so I use it sparingly. Butter freezes REALLY well! I buy the Costco size and cut it into half sticks. Seal it well and keep a chunk to use and the rest gets frozen.

Best of luck to everyone here. My sister has had arthritis since she was a kid. I know how painful it is.