I bought my 06 off road in April 2006. I worked out of town for until 2008 so it didn't get much mileage on it. I now have 43500 since I am now retired and drive it a lot more. Noticed that the fuel economy, If you can call it that in an Xterra, has gone down lately. I am going to run some injector cleaner through it this week and see if that helps.
Your plugs might be getting a bit long in the tooth, and, MAF cleaner might be a good idea too.
Its also possible that you need to change some lubes other than engine oil, like in the diffs, etc.
Going full synthetic saved me a ton o $
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- TJ
2001 Xterra '03 VG33, SE 5 spd, 305/70/16's, Revolvers, UBSkidderz, Doubled AAL's, 3"SL/2"BL, winch/bumpers, skids, sliders, OBA, Snorkel, pine stripes....
Friends don't let friends drive stock.
http://www.gifsoup.com/view/501230/tj-tackling-crawlers-ridge-o.gif