I have done a fair number of fans where you could not replace them easily ( cost, availability and lead times) and the drop of oil worked for a long period, often months or years. Silicone oil is not the best lubricant in a bearing, as it tends to polymerise with time and with temperature, turning into an abrasive. As well the breakdown product is silicon dioxide, AKA sand. Mixed with carbon based oil and you get as a breakdown product silicon carbide, or grinding paste.
The engine oil is light enough to lubricate, but thick enough that it will stay put in the bearing, providing a nice hydrodynamic film to reduce the friction between the sleeve and the shaft. It likely is also the original oil as well. Oilite bushes are typically prepared during manufacture after sintering by dumping them hot into hot oil which both cools them and impregnates them with oil. A lot these days seem to be made from leaded steel powder pressed into a bush then sintered, must be cheaper than bronze or brass powder.