Contrary to popular belief silicone oil is extremely flammable.
The viscosity doesn’t really change a-whole lot during normal serviceability of the oil, the amount of material (steel or other debris) mixed with it goes up and up the longer the differential fluid is not changed this material separates the silicone molecules and reduces viscosity in that manner. Silicone has a property that limits the shear loading aspect and that it is lower than most other lubricants.

Since RC cars nature is to rather contaminate the silicone over a period of time the viscosity will only go so low before other things are obviously wrong.

As long as the silicone oil did not reach vitrification silicone oil can be filtered but who has time for that.

This is why is it absolutely important that the silicone oil used in satellites must be 110% pure and clean room contaminate free.

Excuse me what? Do you have a video of silicone oil bursting into flames in at least semi-realistic circumstances or sth to back that up? I just looked at some product safety datasheets and was only able to find one that listed it as extremely flammable, but that was not pure silicon oil but silicon oil in a pressurized can, where the aerosol was the flammable part. I also may or may not have spent the past 5 minutes holding a small blowtorch into a puddle of silicone oil with no fire to be seen.

Silicone is flammable -- I've set silicone rubber on fire with a blowtorch a couple times, and it's extremely hard to extinguish -- but the temperature required to start combustion is so high that it's irrelevant. In a RC context, nothing short of a large lithium battery fire will cause silicone to catch fire. Silicone oil might actually evaporate before it has a chance to combust, whereas silicone rubber at least has the decency to hold still while you blowtorch it.

Correct. I thought the fact that we were talking about fresh oil was obvious from the fact that its shelf life was being questioned.