i would expect there to be a small amount of oils in there as those were usually used in conjuction with a vacuum assist flap on the inlet manifold on the Series3 diesel 2.25, most likely it will actually be engine oil vapours settled in it
if your servo is plumbed to a vacuum pump then that reservoir isn't required and is just a "leftover" from the original system, you can leave it bolted to the inner wing or remove it to suit yourself when you have a vacuum pump
do NOT throw it away as there's bound to be someone that will want it, i passed my one on to another lad on OLLR who wanted one to complete his system, there would normally also be an extra few bits attached to the accellerator linkage and of course the flap valve on the inlet manifold with the original 2.25 diesel engine to complete this system