This happens to mine, and they are most definitely stainless, not aluminum. Our stainless sink gets the same way. I assume it's from all the minerals in our water. I have found that mixing up some Oxyclean and water and a couple of minutes' soak and a scrub with a plastic scrubber will get rid of it (both from the pan and from the sink). It comes right back, though.
I haven't found that it has any negative impact on the usability of the pans, and doesn't shorten the life significantly if the 14+ year old chef's pan I use almost daily is any indication.
I did a little more research, and it's definitely mineral haze. Recommended treatments are Barkeeper's Friend (mild oxalic acid) soak/scrub, or boiling some vinegar, either one followed by a thorough rinse. In theory, if the haze is left alone it could develop into buildup, which would begin to degrade performance, but in practice it's not an urgent worry.
So if it bugs you, clean it off with mild acid or oxygen bleach. But expect it to come right back. If you only removed it once a year, you'd be a couple of years ahead of my cleaning schedule, and I haven't seen any harm from it.
This is quite common and pretty harmless. The scratches you see don't go very deep, nor are they very wide. My All-Clad saute pan is nearing 10 years old and has a ton of micro-scratches on the interior. It still performs beautifully.
That said, the scratches can grab onto proteins and cause sticking. However, this is simple to prevent with both oil and proper pan preheating.
When a pan is preheated properly the metal expands, essentially closing all of the micro-scratches. This prevents the proteins from grabbing onto them and getting stuck. You obviously need oil/fat to assist with this as well.
To properly heat a pan to the appropriate temperature I suggest using the water drop method. If you put a cold pan on heat and drip a drop of water onto it, the water will sit there for several seconds then boil away. As the pan gets warmer this will happen more quickly, fizzling away in a second or so. Once the scratches start to close something weird happens.
First, the drop of water will break into a few mini drops which scoot around the pan as they evaporate. This is a sign that you are almost there. When the drop of water stays whole (mostly) and scoots around the pan like a mercury ball, this is the perfect temperature. I the water instantly vaporizes on contact, you've gone way too far and need to let the pan cool down. At this point you should add your oil/fat, swirl it around, and immediately add your food. (Make sure the mercury ball of water is gone before adding oil).
Also note that the mercury-ball phase is definitely too hot for unclarified butter, and may be too hot for some extra-virgin olive oils. They may instantly smoke upon adding.
Again, it's important to have your oil and ingredients in place (mise en place) before you start. It's quite easy to skyrocket past the mercury-ball phase if you have to open your oil, pour, and then season your ingredients.
Best Answer
Of course you can. I have often baked pies and tarts in metal dishes: one can even buy metal dishes with removable bottoms specifically for baking tarts.
In order of heat capacity, stainless steel has the least heat capacity, then cast iron, then ceramic. The metal pans are more conductive (and darker) so you will find your crust cooks slightly more quickly. I would bear this in mind particularly if you are blind-baking a pie or tart crust.
I suggest you do not vary the temperature, since this just introduces an extra variable, and will affect how the filling cooks as well as the crust.