Do not used any kind of petroleum based oil. This will over time damage any plastic or rubber parts it contacts. (My X-45 base disintegrated because of this).
I use Sil-Glide... this is a lubricant for automotive brake parts, and is designed to be safe with the rubber boots and seals of brake calipers. You can probably pick some up at a local auto parts store (I'm an auto mechanic and my shop graciously 'donated' me a small tube for the cause).