in my car I really never deviate from either 10w-30 or 10w-40 depending which is on sale. In my moms car however I use 5w-20 in the summer and 10w-40 in the winter.
plus a little bit of lucus.. don't kill me!
oil 101 thanks to our friends at supermania.. give it a read
http://www.supramania.com/aehaas/