How did your oil turn out? Would be curious to hear how it worked. I have used Solomon’s Seal internally (tincture, tea, and chewing the root) to great effect but never externally and would love to hear how it went as it is something I have considered making myself, but am hesitant, given that at least where I live, it is not a terribly common plant.
If you plan to make further oils or salves for this issue, another thing to try adding might be Cottonwood buds. I have not used them myself, but my herbalist friend and mentor suggested them for joint pain.
Perhaps you have already considered this, but tending to the muscles surrounding an achy joint can make a tremendous difference. Gentle stretching, foam rolling and massage are wonderful for this. As a massage therapist, I often find that knee pain is accompanied by tight quadriceps, hamstrings and iliotibial bands, among other things. Working on these muscle groups has often provided significant relief. Epsom salt baths can help quite a bit as well, as most folks do not get
enough magnesium, which is vital to muscle health.