Okay, here comes the silly newbie questions....

Is it safe to leave powder/pellets in a car when it's really hot outside? 100'ish +/- degrees is the range that I'm talking about.

In the event that my muzzleloader has to be stored for an extended period of time between shots is it recommended that I clean and oil it regularly?

Leaving any kind of powder in an enclosed place where it can be subjected to some extreme temperatures, especially heat is not a good idea. I believe there is even a warning on the powder jug to that effect. While I am not saying it will explode, there always is the possibility, so for safety reasons I would not do it...

If you have shot your rifle and will not be shooting it for an extended period of time, then by all means clean the rifle. Some powders like Pyrodex RS for instance can start rusting the bore in a matter of days. If the rifle was subjected to moisture on top of that, it can accelerate the process.

I think I misworded the second question. I do clean it after each shoot that I go to, however, say it's not going to be used for a long period of time 6+ months and I cleaned it when I last shot it. Should I clean it maybe once or twice during the timeframe that it is stored?

I always check my "stored" rifles about every four months and give them a quick going over whether they need it or not. It would not hurt to check the rifle periodically just to make sure all is well, no rust is forming, etc..

1) Not a good idea. At least keep it in the shade and roll your windows down to keep the cab reasonably cool if you feel safe with that.

2) Always check your guns monthly or more, and renew the oil. I did the mistake of waiting 2 months on my Omega and had a, thankfully, very small rust spot and no bore rust. Thanks to catching it quickly I was able to rub the rust awaywith oiled patches.My attention must've been on my hawken too much