synthetic motor oil... which ones are truly synthetic?

I have read some articles online about some so called "synthetic" motor oils, not being 100% synthetic and actually having additives in them.
I was told that penzoil sucks,
what are the good brands that make actual good synthetic oil?
i looked at chevron's website, it says 100% synthetic, is this true?
please post up your thoughts on motor oil because I am due for a oil change soon