Throw them out once you loose the cap? Shame on you!
For those of us who don't have more than a few, an RB44 slips snuggly over the tube to prevent leakage.
30wt is my oil of choice. Actually, any motor oil over the lightweight stuff. My trouble is, I lube those GAL oilers to their death.
I find the lighter lubricants don't last as long on oilers or chicken wing closer hardware, but the lighter penetrating spray lubes work better on pivot points on any door equipment.