CEL Codes p0171 and p0174

Hi Everyone. I have a 2001 Ford Ranger and the CEL has come on and gave me the following codes: p0171 and p0174. They both say "System too lean". I have ran Chevron w/techron and middle grade gas through but the light won't stay off.
Any Ideas?
Thanks in Advance

You may want to change your fuel filter. I if it continues after this, check your fuel pressure. You're obviously not getting fuel to the injectors like you need.

I don't know. I bet you're getting better gas mileage right now, aren't you? :p

well i made a trip recently with it and got around 20.5mpg.

btw, thanks I will try that.