An engineer friend of mine asked, "Why does Nikon use grease to lubricate?"

Tribology is the study of wear and lubrication, so he should be well qualified to ask the question.

I'm not a tribologist but I am a mechanical engineer and I've had a bit of experience with machine lubrication.

Just to clear up a few things:

1.  Grease and oil are not the same things.  Grease is simply oil suspended in a thick porous substance, which is technically known as a soap.  Whether a component is lubricated using grease or oil, it's actually oil that is doing the lubricating in both cases.

2.  Greases are typically used where oil can't be e.g. some high temp applications, or where it's difficult/impossible to contain oil.  Also they tend to be used where contamination is a major problem as the grease itself is a good barrier to dust and other contaminants.

3.  Synthetic oils don't really offer any advantage in terms of viscosity and operating temperature.  They do offer advantages in that they don't break down as easily and retain their properties better under extreme conditions such as high pressure.

4.  Wide operating temp ranges for oils are obtained by mixing grades of oil, and by using different additive packages.

I'm not an expert in camera design but it would surprise me somewhat if they use grease on the shutter mechanism.  Firstly, because of its physical properties, grease is very difficult to fling around.  Secondly, if it was grease, I suspect removing it from the sensor would be much more difficult than it appears to be.