I purchased that 18w bulb--it does work in the Thomas (no, the Thomas bulb does not have a proprietary base), and I'm still using it almost one year later. But I didn't notice much difference on the light output. It was still too bright without cardboard shields covering about half the opening.
One would think that, with half the wattage, there would be a noticeable reduction in light. But at least the bulb is less expensive than the "correct" one from Thomas.