I use linseed oil for wooden and also horn hilts and scabbards of my ethnograhic weapons even for the nice finish (yes, it make the wood/horn darker, see the different here:
http://www.vikingsword.com/vb/showth...ighlight=luzon) and to oil the parts to prevent cracks and don't let become the oiled parts to dry. I don't see any risk by this, think about that many our weapons coming from very hot parts of the world. When a native has taken his weapon in hand he will oil it with his own sweat and remaining oil from food he has eaten before so all sorts of natural oil feeding the wood/horn or also ivory over decades and give it the so demanded patina. So why should linseed oil wrong for this parts?
But frankly said I never have oiled one of my shields.
Regards,
Detlef