Oils have a certain amount of additives to protect the parts from corrosion, but the longer you leave it in the equipment, the higher the risk of problems from the entrance of water or chemicals from the air.
In the first example, we have a stationary generator that did not show any corrosion before being drained for transport to another location. Two months later it was installed in the new location. The first oil sample had 195 ppm of copper corrosion from the oil cooler that was exposed to the air during those 2 months without oil circulating to protect it.
When it finally was taken to the shop, the rust and wear were 964 ppm of iron that could be read in the oil analysis. the bigger particles (over 5 microns) normally do not show up.