Couple of oil questions

[burgerboy]

I'm planning on switching over to Mobile 1 synthetic with Wix or Purelator filter this weekend, but have a couple of questions.

Here in Iowa, the winters gets pretty cold and the summer, while not extremely hot in temperature, is quite humid. What viscoscity do you guys recommend? I was thinking 0w-40.

Is it ok to switch viscoscity based on season? ie, use 'thicker' oil for the summers, and 'thinner' oils for the winter? Or should I pick one and stick with that?

Lastly, the dealership has been handling the oil changes. I got it done at 1000, and 3000 miles, and the car is just over 5000 right now. Is this too early to switch to a synthetic?

TIA!

Oh...when torquing the drain bolt back on, is there a specific torque or does everyone tighten by 'feel?'

[burgerboy]

Due to a storm moving in, I was rushing to get everything done.

Couple of things:

If I didn't losen the filler cap while the oil is draining, will that cause any problems? I tightened the drain plug before changing out the filter, so there were never 2 openings for draining. Will this cause extra oil to not be drained? Which brings me to my next question.

Also, after filling the oil, best guess-ti-mation by the back of the Mobile1 jug, which looked to have half a quart left (4.5 qt in the car), I ran the car for 5 min. and let it sit for 5 min. Checking the dipstick has the oil level right at the full mark. Is this cause for concern? Do I need to drain some out?

Why did I rush???