I have several of these bodies. I love the camera. The previous posters pretty much covered the MLU question. To reiterate, the mirror flips up when you press the shutter and goes back down at the end of the exposure time. MLU is used when you don't want to endure the shake from the mirror. You push the MLU button, the mirror flips up, you press the shutter and the mirror returns after the exposure time.
The 105 lens tends to yellow as it ages. I don't remember if it's a coating or the type of glass that's used but I'm sure someone here will be able to elaborate.
Finally use a good battery. I used to use lithium but silver batteries are better for this camera especially if you are using the metered finder. I can't tell you the number of times that I've tripped the shutter only to have the mirror stay locked up because of a low battery.