I believe the original specifications was the equivalent of a low melting point grease or very heavy oil. Once the engine got hot, this would melt/thin and then wool/felt wick would pull the lubricant up and through the slot to the center of the hollow rocker spindle. This would prevent capillary action from draining the reservoir when the engine was not in use. The downside is no lubricate would flow until the reservoir got up to temperature an would continue to flow until the engine cooled down. But presumably it averaged out! The slot in the rocker spindle only goes about a third to half way down into the depth of the reservoir (if it has not been altered), so just filling the reservoir with lubricant will starve the rocker arms unless you kept them filled right to the top.
If you only did short trips where the reservoirs did not get hot (sprinting), or are worried about initial lubrication until the reservoir got hot, taking the grease fittings out and applying a squirt of oil directly to the center hole of the rocker spindle would probably be a good idea to supliment the grease.
-Doug
