I have a 2003 Jetta TDI with 90k. Until recently I have consistently gotten 46-48 mpg. Over the past 3 months the mpg has been decreasing. The current mpg is about 30. I change the oil every 3k (synthetic), air filter changed 60 days ago, new tires have less than 5k. Any ideas for the decrease in mpg?

Your original post does not specify your location. Significant amounts of short time driving will hurt your MPG, as will giving the engine a hard workout during cold (Less than 50F or 15C is cold to me). this add to the time that the engine gets to operating temp. Check your coolant temp. How long does it take till it is in the 'Normal' operating range. You may have a stuck coolant thermostat (keeps coolant flowing even though engine is cold). Have you changed driving patterns in the past 3mos? is this the 1st Season of ownership?

Oil Change every 3000(miles?)

Check your owners manual. A TDI does not need to have the oil changed every 3K(Miles?). Make sure that the OIL is the right weight for the engine,

Currently TDI specs call for fully synthetic types. See Manual

If the Wrong oil is replaced, then that could explain a less tight seal on the cylinder walls, and a corresponding loss in compression tightness, requiring more fuel for each stroke (A 5% or more additional fuel per stroke)

ONLY if you are a very dusty environment, should you require a 3k oil interval. Change the air cleaner( or shake it out more often). You would be surprised as to how much air is consumed in 3000 miles of driving.

Don't know about the MPG it could be a lot of things. I have a 2001 Jetta TDI. You should NEVER change the oil at 3K miles. VW recommends 10K miles for a reason. The Synthetic oil has additives to remove varnish and it does that but it needs some time to replace the varnish and properly protect sensitive engine parts. 3K is oil changes are not a good thing for a TDI......Just use the right oil and change at 10K