Yes. Too low a density and you would still need to oil them at least every day. Too high a density and you'll damage the bearings. Back before I knew that I could still get some parts from Clausing and other sources, I bought some felt rod material in about the right diameter from McMaster. I cut off two pieces of about the right length and because the things were too hard to just shove into the cups dry, I luckily decided to oil them before shoving them into the cups. I squirted some on one end, and it just sat there. Several hours later, it still wasn't even close to soaking through to the other end.