My SES light used to come on and turn off about every 25 starts or so. For the last 2 months or so it has been on all the time.

When I used to drive 200 miles a day I was getting 25-28 mpg. No that I drive ONLY 100 miles per day it has dropped to around 23-24 mpg.

The only thing I did change was I took out my K&N filter and put the stock Nissan one back in.

I figure maybe that change and perhaps winter gas(don't they put some crap in it to reduce emissions in the winter?) is causing the lower mpg....or is it my SES light?

 

Yes. But it's not the SES light that's reducing your gas mileage. It's what's causing it . . . probably a bad O2 sensor.

 

Another factor to consider is that with colder winter temperatures, it takes longer for an engine to reach its full operating temperature. You did not mention if the reduced number of miles you are now driving is a series of short trips or a back and forth commute. In any event, the longer (and more frequently) an engine has to get to optimum temp, the righer it runs and the lower the mpg.