left when spraying WD-40 or other similar product,that layer will repel water or moist ,I tried spraying some MoSOil from sonax (very similar to WD-40 but with some more additives),after several days I was checking the engine bay and poured water on an area with MoSOil and the water was repelled ,you can see water is forming into droplets. I don't have enough evidence on the ability of how the greasy layer reacts with heat but I think it survived for several days. That was my theory maybe there are other products out their specialized for insulation.