I think the advice of not using too much oil dates to the time in which mineral based oils were not used on keris blades. To my knowledge the oil base was coconut oil with essential oils added. The problem with this oil is that the coconut base can in time go rancid and encourage fungus to grow in the wooden sheath. The digestive enzymes released by the fungus can cause discolouration and damage to the wooden parts of the keris.
I think in ancient times oiling a keris was seen as feeding the blade, remember that many Indonesians believe keris have power.
I remember watching "The ring of fire" on dvd, a scene shot in Indonesia shows some court keris. When the blades have been washed people gather outside the court so that they can wash or even drink some of the water used. They believe that some of the power of the keris will be retained in the water even though it contains arsenic
