Wood is not as dense as it seems. After drying there are pores from evaporation of the water , which make it more or less air permeable depending upon wood, result. Oil is to fill these pores primarily. An instrument from pear-wood e.g. is without oiling so leaky that the deep tones hardly respond. By oiling the instrument becomes more heavy , which has effects on sound and speech. These effects are clearly smaller with hard woods than with soft ones, which can take up more oils. What sort of oil You use is not really important but You shouldn't use often linseed oil, for otherwise there would be developed more and more thicker layers on the inner wall, which are to be removed with difficulty. I use an acidless mineral oil with the viscosity 80 (machine-oil). To improve the smell of the oil I give to 100 ml oil to 1 ml Rosmarien-oil. Oil Your instrument whenever you consider it necessary. The instrument should be dry, i.e.: at least not played for one day. The inner wall should be fully moistened. The part of the oil that did not go into the instrument after some hours, should be wiped out with a dry cotton rag. If you believe to notice that an improvement of sound and speech occurred repeat the whole procedure some days later. If there is no improvment after having oiled an instrument, You should stop oiling for some weeks.